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.*;
  
  public class A {
      //--------------------------INPUT READER--------------------------------//
      static class fs {
          public BufferedReader br;
          StringTokenizer st = new StringTokenizer("");
  
          public fs() {
              this(System.in);
          }
  
          public fs(InputStream is) {
              br = new BufferedReader(new InputStreamReader(is));
          }
  
          String next() {
              while (!st.hasMoreTokens()) {
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
              return st.nextToken();
          }
  
          int ni() {
              return Integer.parseInt(next());
          }
  
          long nl() {
              return Long.parseLong(next());
          }
  
          double nd() {
              return Double.parseDouble(next());
          }
  
          String ns() {
              return next();
          }
  
          int[] na(int n) {
              int[] a = new int[n];
              for (int i = 0; i < n; i++) a[i] = ni();
              return a;
          }
      }
      //-----------------------------------------------------------------------//
  
      //---------------------------PRINTER-------------------------------------//
      static class Printer {
          static PrintWriter w;
  
          public Printer() {
              this(System.out);
          }
  
          public Printer(OutputStream os) {
              w = new PrintWriter(os);
          }
  
          public void p(int i) {
              w.println(i);
          }
  
          ;
  
          public void p(long l) {
              w.println(l);
          }
  
          ;
  
          public void p(double d) {
              w.println(d);
          }
  
          ;
  
          public void p(String s) {
              w.println(s);
          }
  
          ;
  
          public void pr(int i) {
              w.println(i);
          }
  
          ;
  
          public void pr(long l) {
              w.print(l);
          }
  
          ;
  
          public void pr(double d) {
              w.print(d);
          }
  
          ;
  
          public void pr(String s) {
              w.print(s);
          }
  
          ;
  
          public void pl() {
              w.println();
          }
  
          ;
  
          public void close() {
              w.close();
          }
  
          ;
      }
      //------------------------------------------------------------------------//
  
      //--------------------------VARIABLES------------------------------------//
      static fs sc = new fs();
      static OutputStream outputStream = System.out;
      static Printer w = new Printer(outputStream);
      //-----------------------------------------------------------------------//
  
      //--------------------------ADMIN_MODE-----------------------------------//
      private static void ADMIN_MODE() throws IOException {
          if (System.getProperty("ONLINE_JUDGE") == null) {
              w = new Printer(new FileOutputStream("output.txt"));
              sc = new fs(new FileInputStream("input.txt"));
          }
      }
      //-----------------------------------------------------------------------//
  
      //----------------------------START--------------------------------------//
      public static void main(String[] args)
              throws IOException {
  
          ADMIN_MODE();
  
          int t = sc.ni();
          while (t-- > 0)
              solve();
  
  
          w.close();
      }
  
      static void solve() throws IOException {
          int a = sc.ni();
          int b = sc.ni();
          int diff = a - b;
          if(a == b && a == 0) {
              w.p(0);
              return;
          }
          if (diff % 2 != 0) {
              w.p(-1);
              return;
          }
          if(a == b) {
              w.p(1);
              return;
          }
          w.p(2);
      }
  }