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.InputStream;
  import java.io.InputStreamReader;
  import java.util.StringTokenizer;
  
  public class A {
      
      public static void main(String[] args) throws IOException {
          reader.init(System.in);
          int t = reader.nextInt();
          for (int o = 0; o < t; o++) {
              int c = reader.nextInt(), d = reader.nextInt();
              int a = 0, b = 0;
              if(abs(c - d) % 2 != 0) {
                  System.out.println(-1);
              }
              else if(c == 0 && d == 0) {
                  System.out.println(0);
              }
              else if(c == d) {
                  System.out.println(1);
              }
              else {
                  System.out.println(2);
              }
          }
      }
      
      
      
      
      
      
      
      
      
      static class reader {
          static BufferedReader reader;
          static StringTokenizer tokenizer;
          static void init(InputStream input) {
              reader = new BufferedReader(new InputStreamReader(input));
              tokenizer = new StringTokenizer("");
          }
          static String next() throws IOException {
              while (!tokenizer.hasMoreTokens()) {
                  tokenizer = new StringTokenizer(reader.readLine());
              }
              return tokenizer.nextToken();
          }
          static int nextInt() throws IOException {return Integer.parseInt(next());}
          static double nextDouble() throws IOException {return Double.parseDouble(next());}
          static long nextLong() throws IOException {return Long.parseLong(next());}
      }
      static long min(long a, long b) {return Math.min(a, b);}
      static long min(long a, long b, long c) {return min(a, min(b, c));}
      static int min(int a, int b) {return Math.min(a, b);} 
      static int min(int a, int b, int c) {return min(a, min(b, c));}
      static long max(long a, long b) {return Math.max(a, b);}
      static long max(long a, long b, long c) {return max(a, max(b, c));}
      static int max(int a, int b) {return Math.max(a, b);}
      static int max(int a, int b, int c) {return max(a, max(b, c));}
      static int abs(int x) {return Math.abs(x);} 
      static long abs(long x) {return Math.abs(x);} 
      static double sqrt(int x) {return Math.sqrt(x);}  
      static int gcd(int a, int b) {if(b == 0)return a;return gcd(b, a % b);}
  
  }
  