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.Arrays;
  import java.util.PriorityQueue;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Scanner;
  import java.util.Stack;
  import java.util.StringTokenizer;
   
  public class Main
  {
  static class FastReader
  {
  BufferedReader br;
  StringTokenizer st;
  public FastReader()
  {
  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;
  }
  }
   
  
  public static void main(String[] args)
  {
         FastReader in = new FastReader();
         int test=in.nextInt();
         while(test-->0)
         {
      	   int a=in.nextInt();
      	   int b=in.nextInt();
      	   
      	   if(a==0&&b==0)
      		   System.out.println(0);
      	   else
      		   if(a==b)
      			   System.out.println(1);
      		   else
      	   if(Math.abs(a-b)>=2&&(a+b)%2==0)
      		   System.out.println(2);
      	   else
      		   System.out.println(-1);
         }
  }
  }