description: <image>
  
  William has two numbers a and b initially both equal to zero. William mastered performing three different operations with them quickly. Before performing each operation some positive integer k is picked, which is then used to perform one of the following operations: (note, that for each operation you can choose a new positive integer k)
  
    1. add number k to both a and b, or 
    2. add number k to a and subtract k from b, or 
    3. add number k to b and subtract k from a. 
  
  
  
  Note that after performing operations, numbers a and b may become negative as well.
  
  William wants to find out the minimal number of operations he would have to perform to make a equal to his favorite number c and b equal to his second favorite number d.
  
  Input
  
  Each test contains multiple test cases. The first line contains the number of test cases t (1 ≤ t ≤ 10^4). Description of the test cases follows.
  
  The only line of each test case contains two integers c and d (0 ≤ c, d ≤ 10^9), which are William's favorite numbers and which he wants a and b to be transformed into.
  
  Output
  
  For each test case output a single number, which is the minimal number of operations which William would have to perform to make a equal to c and b equal to d, or -1 if it is impossible to achieve this using the described operations.
  
  Example
  
  Input
  
  
  6
  1 2
  3 5
  5 3
  6 6
  8 0
  0 0
  
  
  Output
  
  
  -1
  2
  2
  1
  2
  0
  
  Note
  
  Let us demonstrate one of the suboptimal ways of getting a pair (3, 5):
  
    * Using an operation of the first type with k=1, the current pair would be equal to (1, 1). 
    * Using an operation of the third type with k=8, the current pair would be equal to (-7, 9). 
    * Using an operation of the second type with k=7, the current pair would be equal to (0, 2). 
    * Using an operation of the first type with k=3, the current pair would be equal to (3, 5). 
solution: //package codeforces;
  import java.io.*;
  import java.util.ArrayDeque;
  import java.util.Arrays;
  import java.util.InputMismatchException;
  import java.util.Queue;
  
  public class Solution {
      InputStream is;
      FastWriter out;
      String INPUT = "";
  
      void solve() {
          for (int T = ni(); T > 0; T--) {
              go();
          }
      }
  
      void go() {
          int c = ni(), d = ni();
          if (c == 0 && d == 0) {
              out.println("0");
          } else if (c == d) {
              out.println("1");
          } else if ((c + d) % 2 == 0) {
              out.println("2");
          } else {
              out.println("-1");
          }
      }
  
      void run() throws Exception
      {
          is = oj ? System.in : new ByteArrayInputStream(INPUT.getBytes());
          out = new FastWriter(System.out);
  
          long s = System.currentTimeMillis();
          solve();
          out.flush();
          tr(System.currentTimeMillis()-s+"ms");
      }
  
      public static void main(String[] args) throws Exception { new Solution().run(); }
  
      private byte[] inbuf = new byte[1024];
      public int lenbuf = 0, ptrbuf = 0;
  
      private int readByte()
      {
          if(lenbuf == -1)throw new InputMismatchException();
          if(ptrbuf >= lenbuf){
              ptrbuf = 0;
              try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
              if(lenbuf <= 0)return -1;
          }
          return inbuf[ptrbuf++];
      }
  
      private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
      private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
  
      private double nd() { return Double.parseDouble(ns()); }
      private char nc() { return (char)skip(); }
  
      private String ns()
      {
          int b = skip();
          StringBuilder sb = new StringBuilder();
          while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ')
              sb.appendCodePoint(b);
              b = readByte();
          }
          return sb.toString();
      }
  
      private char[] ns(int n)
      {
          char[] buf = new char[n];
          int b = skip(), p = 0;
          while(p < n && !(isSpaceChar(b))){
              buf[p++] = (char)b;
              b = readByte();
          }
          return n == p ? buf : Arrays.copyOf(buf, p);
      }
  
      private int[] na(int n)
      {
          int[] a = new int[n];
          for(int i = 0;i < n;i++)a[i] = ni();
          return a;
      }
  
      private long[] nal(int n)
      {
          long[] a = new long[n];
          for(int i = 0;i < n;i++)a[i] = nl();
          return a;
      }
  
      private char[][] nm(int n, int m) {
          char[][] map = new char[n][];
          for(int i = 0;i < n;i++)map[i] = ns(m);
          return map;
      }
  
      private int[][] nmi(int n, int m) {
          int[][] map = new int[n][];
          for(int i = 0;i < n;i++)map[i] = na(m);
          return map;
      }
  
      private int ni() { return (int)nl(); }
  
      private long nl()
      {
          long num = 0;
          int b;
          boolean minus = false;
          while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
          if(b == '-'){
              minus = true;
              b = readByte();
          }
  
          while(true){
              if(b >= '0' && b <= '9'){
                  num = num * 10 + (b - '0');
              }else{
                  return minus ? -num : num;
              }
              b = readByte();
          }
      }
  
      public static class FastWriter
      {
          private static final int BUF_SIZE = 1<<13;
          private final byte[] buf = new byte[BUF_SIZE];
          private final OutputStream out;
          private int ptr = 0;
  
          private FastWriter(){out = null;}
  
          public FastWriter(OutputStream os)
          {
              this.out = os;
          }
  
          public FastWriter(String path)
          {
              try {
                  this.out = new FileOutputStream(path);
              } catch (FileNotFoundException e) {
                  throw new RuntimeException("FastWriter");
              }
          }
  
          public FastWriter write(byte b)
          {
              buf[ptr++] = b;
              if(ptr == BUF_SIZE)innerflush();
              return this;
          }
  
          public FastWriter write(char c)
          {
              return write((byte)c);
          }
  
          public FastWriter write(char[] s)
          {
              for(char c : s){
                  buf[ptr++] = (byte)c;
                  if(ptr == BUF_SIZE)innerflush();
              }
              return this;
          }
  
          public FastWriter write(String s)
          {
              s.chars().forEach(c -> {
                  buf[ptr++] = (byte)c;
                  if(ptr == BUF_SIZE)innerflush();
              });
              return this;
          }
  
          private static int countDigits(int l) {
              if (l >= 1000000000) return 10;
              if (l >= 100000000) return 9;
              if (l >= 10000000) return 8;
              if (l >= 1000000) return 7;
              if (l >= 100000) return 6;
              if (l >= 10000) return 5;
              if (l >= 1000) return 4;
              if (l >= 100) return 3;
              if (l >= 10) return 2;
              return 1;
          }
  
          public FastWriter write(int x)
          {
              if(x == Integer.MIN_VALUE){
                  return write((long)x);
              }
              if(ptr + 12 >= BUF_SIZE)innerflush();
              if(x < 0){
                  write((byte)'-');
                  x = -x;
              }
              int d = countDigits(x);
              for(int i = ptr + d - 1;i >= ptr;i--){
                  buf[i] = (byte)('0'+x%10);
                  x /= 10;
              }
              ptr += d;
              return this;
          }
  
          private static int countDigits(long l) {
              if (l >= 1000000000000000000L) return 19;
              if (l >= 100000000000000000L) return 18;
              if (l >= 10000000000000000L) return 17;
              if (l >= 1000000000000000L) return 16;
              if (l >= 100000000000000L) return 15;
              if (l >= 10000000000000L) return 14;
              if (l >= 1000000000000L) return 13;
              if (l >= 100000000000L) return 12;
              if (l >= 10000000000L) return 11;
              if (l >= 1000000000L) return 10;
              if (l >= 100000000L) return 9;
              if (l >= 10000000L) return 8;
              if (l >= 1000000L) return 7;
              if (l >= 100000L) return 6;
              if (l >= 10000L) return 5;
              if (l >= 1000L) return 4;
              if (l >= 100L) return 3;
              if (l >= 10L) return 2;
              return 1;
          }
  
          public FastWriter write(long x)
          {
              if(x == Long.MIN_VALUE){
                  return write("" + x);
              }
              if(ptr + 21 >= BUF_SIZE)innerflush();
              if(x < 0){
                  write((byte)'-');
                  x = -x;
              }
              int d = countDigits(x);
              for(int i = ptr + d - 1;i >= ptr;i--){
                  buf[i] = (byte)('0'+x%10);
                  x /= 10;
              }
              ptr += d;
              return this;
          }
  
          public FastWriter write(double x, int precision)
          {
              if(x < 0){
                  write('-');
                  x = -x;
              }
              x += Math.pow(10, -precision)/2;
              //		if(x < 0){ x = 0; }
              write((long)x).write(".");
              x -= (long)x;
              for(int i = 0;i < precision;i++){
                  x *= 10;
                  write((char)('0'+(int)x));
                  x -= (int)x;
              }
              return this;
          }
  
          public FastWriter writeln(char c){
              return write(c).writeln();
          }
  
          public FastWriter writeln(int x){
              return write(x).writeln();
          }
  
          public FastWriter writeln(long x){
              return write(x).writeln();
          }
  
          public FastWriter writeln(double x, int precision){
              return write(x, precision).writeln();
          }
  
          public FastWriter write(int... xs)
          {
              boolean first = true;
              for(int x : xs) {
                  if (!first) write(' ');
                  first = false;
                  write(x);
              }
              return this;
          }
  
          public FastWriter write(long... xs)
          {
              boolean first = true;
              for(long x : xs) {
                  if (!first) write(' ');
                  first = false;
                  write(x);
              }
              return this;
          }
  
          public FastWriter writeln()
          {
              return write((byte)'\n');
          }
  
          public FastWriter writeln(int... xs)
          {
              return write(xs).writeln();
          }
  
          public FastWriter writeln(long... xs)
          {
              return write(xs).writeln();
          }
  
          public FastWriter writeln(char[] line)
          {
              return write(line).writeln();
          }
  
          public FastWriter writeln(char[]... map)
          {
              for(char[] line : map)write(line).writeln();
              return this;
          }
  
          public FastWriter writeln(String s)
          {
              return write(s).writeln();
          }
  
          private void innerflush()
          {
              try {
                  out.write(buf, 0, ptr);
                  ptr = 0;
              } catch (IOException e) {
                  throw new RuntimeException("innerflush");
              }
          }
  
          public void flush()
          {
              innerflush();
              try {
                  out.flush();
              } catch (IOException e) {
                  throw new RuntimeException("flush");
              }
          }
  
          public FastWriter print(byte b) { return write(b); }
          public FastWriter print(char c) { return write(c); }
          public FastWriter print(char[] s) { return write(s); }
          public FastWriter print(String s) { return write(s); }
          public FastWriter print(int x) { return write(x); }
          public FastWriter print(long x) { return write(x); }
          public FastWriter print(double x, int precision) { return write(x, precision); }
          public FastWriter println(char c){ return writeln(c); }
          public FastWriter println(int x){ return writeln(x); }
          public FastWriter println(long x){ return writeln(x); }
          public FastWriter println(double x, int precision){ return writeln(x, precision); }
          public FastWriter print(int... xs) { return write(xs); }
          public FastWriter print(long... xs) { return write(xs); }
          public FastWriter println(int... xs) { return writeln(xs); }
          public FastWriter println(long... xs) { return writeln(xs); }
          public FastWriter println(char[] line) { return writeln(line); }
          public FastWriter println(char[]... map) { return writeln(map); }
          public FastWriter println(String s) { return writeln(s); }
          public FastWriter println() { return writeln(); }
      }
  
      public void trnz(int... o)
      {
          for(int i = 0;i < o.length;i++)if(o[i] != 0)System.out.print(i+":"+o[i]+" ");
          System.out.println();
      }
  
      // print ids which are 1
      public void trt(long... o)
      {
          Queue<Integer> stands = new ArrayDeque<>();
          for(int i = 0;i < o.length;i++){
              for(long x = o[i];x != 0;x &= x-1)stands.add(i<<6|Long.numberOfTrailingZeros(x));
          }
          System.out.println(stands);
      }
  
      public void tf(boolean... r)
      {
          for(boolean x : r)System.out.print(x?'#':'.');
          System.out.println();
      }
  
      public void tf(boolean[]... b)
      {
          for(boolean[] r : b) {
              for(boolean x : r)System.out.print(x?'#':'.');
              System.out.println();
          }
          System.out.println();
      }
  
      public void tf(long[]... b)
      {
          if(INPUT.length() != 0) {
              for (long[] r : b) {
                  for (long x : r) {
                      for (int i = 0; i < 64; i++) {
                          System.out.print(x << ~i < 0 ? '#' : '.');
                      }
                  }
                  System.out.println();
              }
              System.out.println();
          }
      }
  
      public void tf(long... b)
      {
          if(INPUT.length() != 0) {
              for (long x : b) {
                  for (int i = 0; i < 64; i++) {
                      System.out.print(x << ~i < 0 ? '#' : '.');
                  }
              }
              System.out.println();
          }
      }
  
      private boolean oj = System.getProperty("ONLINE_JUDGE") != null;
      private void tr(Object... o) { if(!oj)System.out.println(Arrays.deepToString(o)); }
  }
  