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: import java.io.*;
  import java.util.*;
  import java.math.*;
  import java.lang.*;
  
  public class A_A_Variety_of_Operations implements Runnable {
      static class InputReader {
          private InputStream stream;
          private byte[] buf = new byte[1024];
          private int curChar;
          private int numChars;
          private SpaceCharFilter filter;
          private BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  
          public InputReader(InputStream stream) {
              this.stream = stream;
          }
  
          public int read() {
              if (numChars == -1)
                  throw new InputMismatchException();
  
              if (curChar >= numChars) {
                  curChar = 0;
                  try {
                      numChars = stream.read(buf);
                  } catch (IOException e) {
                      throw new InputMismatchException();
                  }
  
                  if (numChars <= 0)
                      return -1;
              }
              return buf[curChar++];
          }
  
          public String nextLine() {
              String str = "";
              try {
                  str = br.readLine();
              } catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
  
          public int nextInt() {
              int c = read();
  
              while (isSpaceChar(c))
                  c = read();
  
              int sgn = 1;
  
              if (c == '-') {
                  sgn = -1;
                  c = read();
              }
  
              int res = 0;
              do {
                  if (c < '0' || c > '9')
                      throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              } while (!isSpaceChar(c));
  
              return res * sgn;
          }
  
          public long nextLong() {
              int c = read();
              while (isSpaceChar(c))
                  c = read();
              int sgn = 1;
              if (c == '-') {
                  sgn = -1;
                  c = read();
              }
              long res = 0;
  
              do {
                  if (c < '0' || c > '9')
                      throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              } while (!isSpaceChar(c));
              return res * sgn;
          }
  
          public double nextDouble() {
              int c = read();
              while (isSpaceChar(c))
                  c = read();
              int sgn = 1;
              if (c == '-') {
                  sgn = -1;
                  c = read();
              }
              double res = 0;
              while (!isSpaceChar(c) && c != '.') {
                  if (c == 'e' || c == 'E')
                      return res * Math.pow(10, nextInt());
                  if (c < '0' || c > '9')
                      throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              }
              if (c == '.') {
                  c = read();
                  double m = 1;
                  while (!isSpaceChar(c)) {
                      if (c == 'e' || c == 'E')
                          return res * Math.pow(10, nextInt());
                      if (c < '0' || c > '9')
                          throw new InputMismatchException();
                      m /= 10;
                      res += (c - '0') * m;
                      c = read();
                  }
              }
              return res * sgn;
          }
  
          public String readString() {
              int c = read();
              while (isSpaceChar(c))
                  c = read();
              StringBuilder res = new StringBuilder();
              do {
                  res.appendCodePoint(c);
                  c = read();
              } while (!isSpaceChar(c));
  
              return res.toString();
          }
  
          public boolean isSpaceChar(int c) {
              if (filter != null)
                  return filter.isSpaceChar(c);
              return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
          }
  
          public String next() {
              return readString();
          }
  
          public interface SpaceCharFilter {
              public boolean isSpaceChar(int ch);
          }
      }
  
      public static void main(String args[]) throws Exception {
          new Thread(null, new A_A_Variety_of_Operations(), "Main", 1 << 27).start();
      }
  
      static class Pair {
          int f;
          int s;
          int p;
          PrintWriter w;
  
          // int t;
  
          Pair(int f, int s) {
              // Pair(int f,int s, PrintWriter w){
              this.f = f;
              this.s = s;
              // this.p = p;
              // this.w = w;
              // this.t = t;
  
          }
  
          public static Comparator<Pair> wc = new Comparator<Pair>() {
              public int compare(Pair e1,Pair e2){
  
                  // 1 for swap
                  if(Math.abs(e1.f)-Math.abs(e2.f)!=0){
                      // e1.w.println("**"+e1.f+" "+e2.f);
                      return (Math.abs(e1.f)-Math.abs(e2.f));
                  }
                  else{
                      // e1.w.println("##"+e1.f+" "+e2.f);
                      return (Math.abs(e1.s)-Math.abs(e2.s));
                  }
  
  
              }
          };
      }
  
      public Integer[] sort(Integer[] a) {
          Arrays.sort(a);
          return a;
      }
  
      public Long[] sort(Long[] a) {
          Arrays.sort(a);
          return a;
      }
  
      public static ArrayList<Integer> sieve(int N) {
          int i, j, flag;
  
          ArrayList<Integer> p = new ArrayList<Integer>();
  
          for (i = 1; i < N; i++) {
  
              if (i == 1 || i == 0)
                  continue;
  
              flag = 1;
  
              for (j = 2; j <= i / 2; ++j) {
                  if (i % j == 0) {
                      flag = 0;
                      break;
                  }
              }
              if (flag == 1) {
                  p.add(i);
              }
          }
          return p;
      }
  
      public static long gcd(long a, long b) {
          if (b == 0)
              return a;
          else
              return gcd(b, a % b);
      }
  
      public static int gcd(int a, int b) {
          if (b == 0)
              return a;
          else
              return gcd(b, a % b);
      }
  
      //// recursive dfs
      public static int dfs(int s, ArrayList<Integer>[] g, long[] dist, boolean[] v, PrintWriter w, int p) {
          v[s] = true;
          int ans = 1;
          // int n = dist.length - 1;
          int t = g[s].size();
          // int max = 1;
          for (int i = 0; i < t; i++) {
              int x = g[s].get(i);
              if (!v[x]) {
                  // dist[x] = dist[s] + 1;
                  ans = Math.min(ans, dfs(x, g, dist, v, w, s));
              } else if (x != p) {
                  // w.println("* " + s + " " + x + " " + p);
                  ans = 0;
              }
          }
          // max = Math.max(max,(n-p));
          return ans;
      }
  
      //// iterative BFS
      public static int bfs(int s, ArrayList<Integer>[] g, long[] dist, boolean[] b, PrintWriter w, int p) {
          b[s] = true;
          int siz = 1;
          // dist--;
          Queue<Integer> q = new LinkedList<>();
          q.add(s);
          while (q.size() != 0) {
              int i = q.poll();
              Iterator<Integer> it = g[i].listIterator();
              int z = 0;
              while (it.hasNext()) {
                  z = it.next();
                  if (!b[z]) {
                      b[z] = true;
                      // dist--;
                      dist[z] = dist[i] + 1;
                      // siz++;
                      q.add(z);
                  } else if (z != p) {
                      siz = 0;
                  }
              }
  
          }
          return siz;
      }
  
      public static int lower(int a[], int x) { // x is the target value or key
          int l = -1, r = a.length;
          while (l + 1 < r) {
              int m = (l + r) >>> 1;
              if (a[m] >= x)
                  r = m;
              else
                  l = m;
          }
          return r;
      }
  
      public static int upper(int a[], int x) {// x is the key or target value
          int l = -1, r = a.length;
          while (l + 1 < r) {
              int m = (l + r) >>> 1;
              if (a[m] <= x)
                  l = m;
              else
                  r = m;
          }
          return l + 1;
      }
  
      public static int lower(ArrayList<Integer> a, int x) { // x is the target value or key
          int l = -1, r = a.size();
          while (l + 1 < r) {
              int m = (l + r) >>> 1;
              if (a.get(m) >= x)
                  r = m;
              else
                  l = m;
          }
          return r;
      }
  
      public static int upper(ArrayList<Integer> a, int x) {// x is the key or target value
          int l = -1, r = a.size();
          while (l + 1 < r) {
              int m = (l + r) >>> 1;
              if (a.get(m) <= x)
                  l = m;
              else
                  r = m;
          }
          return l + 1;
      }
  
      public static long power(long x, long y, long m) {
          if (y == 0)
              return 1;
  
          long p = power(x, y / 2, m) % m;
          p = (p * p) % m;
  
          if (y % 2 == 0)
              return p;
          else
              return (x * p) % m;
      }
  
      public void yesOrNo(boolean f) {
          if (f) {
              w.println("YES");
          } else {
              w.println("NO");
          }
      }
          public boolean isPrime(int n)
          {
              if (n <= 1)
                  return false;
              if (n <= 3)
                  return true;
              if (n % 2 == 0 || n % 3 == 0)
                  return false;
              for (int i = 5; i * i <= n; i = i + 6)
                  if (n % i == 0 || n % (i + 2) == 0)
                      return false;
        
              return true;
          }
      
  
      // Arrays.sort(rooms, (room1, room2) -> room1[1] == room2[1] ? room1[0]-room2[0]
      // : room2[1]-room1[1]);
      // Arrays.sort(A, (a, b) -> Integer.compare(a[0] , b[0]));
      //////////////////////////////////////////////////////////////////////////////////////////////////////
      //////////////////////////////////////////////////////////////////////////////////////////////////////
      // code here
      int oo = (int) 1e9;
      int[] parent;
      int[] dist;
      int[][] val;
      boolean[][] vis;
      ArrayList<Integer>[] g;
      // int[] col;
      // HashMap<Long, Boolean>[] dp;
      //char[][] g;
      // boolean[][] v;
      Long[] a;
      // ArrayList<Integer[]> a;
      // int[][] ans;
      long[][] dp;
      long mod;
      int n;
      int m;
      int k;
      long[][] pre;
      // StringBuilder[] a;
      // StringBuilder[] b;
      // StringBuilder ans;
      int[][] col;
      int[][] row;
      PrintWriter w = new PrintWriter(System.out);
  
      public void run() {
  
          InputReader sc = new InputReader(System.in);
  
          int defaultValue = 0;
          mod =  1000000007;
  
          int test = 1;
          test = sc.nextInt();
          while (test-- > 0) {
           int a =sc.nextInt();
           int b = sc.nextInt();
  
           if(a==0 && b==0)
            w.println(0);
          else if(a==b)
           w.println(1);
           else if((a+b)%2==0)
           w.println(2);
           else
            w.println(-1);
          
         }
  
          w.flush();
          w.close();
  
      }
     
  }
  
  