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.*;
  
  public class Solution {
  
      public static void main(String[] args) throws IOException {
  
          BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  
          int t = Integer.parseInt(br.readLine());
  
          BufferedWriter output = new BufferedWriter(new OutputStreamWriter(System.out));
  
          StringTokenizer st;
  
          while (t-- > 0) {
  
              // int n = Integer.parseInt(br.readLine());
  
              st = new StringTokenizer(br.readLine());
              int c = Integer.parseInt(st.nextToken());
              int d = Integer.parseInt(st.nextToken());
  
              if (c == 0 && d == 0) {
                  output.write("0\n");
                  continue;
              }
  
              int diff =(int)Math.abs(c - d);
  
              if(diff % 2 == 1)
              {
                  output.write("-1\n");
                  continue;
              }
  
              if(diff == 0)
              {
                  output.write("1\n");
                  continue;   
              }
  
              output.write("2\n");
  
              // int arr[] = new int[n];
  
              // for(int i = 0; i < n; i++)
              // {
              // int e = Integer.parseInt(st.nextToken());
              // arr[i] = e;
              // }
  
              // char arr[] = br.readLine().toCharArray();
  
              // output.write();
              // int n = Integer.parseInt(st.nextToken());
  
              // HashMap<Character, Integer> map = new HashMap<Character, Integer>();
  
              // if
              // output.write("YES\n");
              // else
              // output.write("NO\n");
  
              // long a = Long.parseLong(st.nextToken());
              // long b = Long.parseLong(st.nextToken());
  
              // if(flag == 1)
              // output.write("NO\n");
              // else
              // output.write("YES\n" + x + " " + y + " " + z + "\n");
  
              // output.write(n+ "\n");
  
          }
  
          output.flush();
  
      }
  
  }