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.util.StringTokenizer;
  
  public class A {
  
  	public static void main(String[] args) {
  		FastScanner in = new FastScanner();
  		int tt = in.nextInt();
  		for(int pp = 0; pp < tt; pp++) {
              int a = in.nextInt();
              int b = in.nextInt();
              if(Math.abs(a - b) % 2 == 0){
                  if(a - b == 0){
                      if(a == 0){
                          System.out.println(0);
                      }
                      else{
                          System.out.println(1);
                      }
                  }
                  else{
                      System.out.println(2);
                  }
              }
              else{
                  System.out.println(-1);
              }
  		}
  		
  	}
  	
  	static class FastScanner
      {
          BufferedReader br;
          StringTokenizer st;
   
          public FastScanner()
          {
              br = new BufferedReader(new
                      InputStreamReader(System.in));
          }
   
          String next()
          {
              while (st == null || !st.hasMoreElements())
              {
                  try
                  {
                      st = new StringTokenizer(br.readLine());
                  }
                  catch (IOException  e)
                  {
                      e.printStackTrace();
                  }
              }
              return st.nextToken();
          }
          int nextInt()
          {
              return Integer.parseInt(next());
          }
   
          long nextLong()
          {
              return Long.parseLong(next());
          }
   
          double nextDouble()
          {
              return Double.parseDouble(next());
          }
          String nextLine()
          {
              String str = "";
              try
              {
                  str = br.readLine();
              }
              catch (IOException e)
              {
                  e.printStackTrace();
              }
              return str;
          }
   
          int [] readintarray(int n) {
              int res [] = new int [n];
              for(int i = 0; i<n; i++)res[i] = nextInt();
              return res;
          }
          long [] readlongarray(int n) {
              long res [] = new long [n];
              for(int i = 0; i<n; i++)res[i] = nextLong();
              return res;
          }
      }
   
  }
  
  
  