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.util.*;
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.*;
  import java.math.*;
  import static java.lang.Math.min;
  import static java.lang.Math.max;
  import static java.lang.Math.abs;
  
  /* https://codeforces.com/contest/1556/problem/A*/
  public class a1566A {
      static FastScanner fs = new FastScanner();
      static PrintWriter fop = new PrintWriter(System.out);
      static StringBuilder sb = new StringBuilder();
  
      public static void main(String[] args) throws IOException {
          try {
              // solve();
              task();
          } catch (Exception e) {
              return;
          }
          // task();
      }
  
      static void task() throws IOException {
          int t = fs.nextInt();
          while (t-- > 0) {
              int a = fs.nextInt(), b = fs.nextInt();
              if(a==0 && b==0){
                  fop.println(0);
              }
              else if(a==b){
                  fop.println(1);
              }
              else if((a+b)%2==0){
                  fop.println(2);
              }
              else{
                  fop.println(-1);
              }
          }
          fop.close();
          fop.flush();
      }
  
      static void solve() throws IOException {
          int T = fs.nextInt();
          while (T-- > 0) {
              long a = fs.nextLong(), b = fs.nextLong();
              if (b < a * 2) {
                  fop.println(b - a);
  
              } else {
                  fop.println((b - 1) / 2);
              }
  
              // fop.println(ans);
  
          }
          fop.flush();
          fop.close();
      }
  
      static int gcd(int a, int b) {
          if (b > a) {
              int tenp = b;
              b = a;
              a = tenp;
          }
          int temp = 0;
          while (b != 0) {
              a %= b;
              temp = b;
              b = a;
              a = temp;
          }
          return a;
      }
  
      static int setBits(int n) {
  
          if (n == 0)
              return 0;
          return (n & 1) + setBits(n >> 1);
      }
  
      static class FastScanner {
          BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
          StringTokenizer st = new StringTokenizer("");
  
          String next() {
              while (!st.hasMoreTokens())
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              return st.nextToken();
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          int[] readArray(int n) {
              int[] a = new int[n];
              for (int i = 0; i < n; i++)
                  a[i] = nextInt();
              return a;
          }
  
          long[] readLongArray(int n) {
              long[] a = new long[n];
              for (int i = 0; i < n; i++)
                  a[i] = nextLong();
              return a;
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
      }
  }
  