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.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.OutputStreamWriter;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  import java.util.*;
  import java.lang.*;
  import java.io.*;
  public class GFG {
  
    static BufferedReader br;
    static PrintWriter out;
    static StringTokenizer st;
    
  
    public static void main(String[] args) throws IOException {
      br = new BufferedReader(new InputStreamReader(System.in));
      out = new PrintWriter(new OutputStreamWriter(System.out));
      
      int n = nextInt();
      while(n-->0){
          double c = nextDouble();
          double d = nextDouble();
          if(c==0&&d==0){
              out.println(0);
              continue;
          }
          if(c==d){
              out.println(1);
              continue;
          }
          double x = (c+d)/2;
          int y = (int)x;
          
          if(x==y){
               out.println(2);
               continue;
          }
          out.println(-1);
          
      }
      
      
      
      out.close();
    }
  
    static String next() throws IOException {
      while (st == null || !st.hasMoreTokens())
        st = new StringTokenizer(br.readLine().trim());
      return st.nextToken();
    }
  
    static long nextLong() throws IOException {
      return Long.parseLong(next());
    }
  
    static int nextInt() throws IOException {
      return Integer.parseInt(next());
    }
  
    static double nextDouble() throws IOException {
      return Double.parseDouble(next());
    }
  
    static char nextCharacter() throws IOException {
      return next().charAt(0);
    }
  
    static String nextLine() throws IOException {
      return br.readLine().trim();
    }
  }