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.*;
  import java.util.*;
  
  public class deltix_a {
  	public static void main(String args[]){
  		FScanner in = new FScanner();
  		PrintWriter out = new PrintWriter(System.out);
  		int t = in.nextInt();
  		while(t-->0) {
           int c=in.nextInt();
           int d=in.nextInt();
           if(c==0 && d==0)
           	out.println(0);
           else if(c==d)
           	out.println(1);
           else if((c+d)%2==0)
           	out.println(2);
           else 
           	out.println(-1);
  		 }
  		out.close();
  	}
  
      static class FScanner {
  		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  		StringTokenizer sb = new StringTokenizer("");
   
  		String next(){
  			while(!sb.hasMoreTokens()){
  				try{
  					sb = new StringTokenizer(br.readLine());
  				} catch(IOException e){ }
  			}
  			return sb.nextToken();
  		}
  		String nextLine(){
  			try{ return br.readLine(); } 
  			catch(IOException e) { } return "";
  		}
   
  		int nextInt(){
  			return Integer.parseInt(next());
  		}
   
  		long nextLong() {
  			return Long.parseLong(next());
  		}
   
  		int[] readArray(int n) {
  			int a[] = new int[n];
  			for(int i=0;i<n;i++) a[i] = nextInt();
  			return a;
  		}
   
  		float nextFloat(){
  		return Float.parseFloat(next());
  		}
  		
  		double nextDouble(){
  		return Double.parseDouble(next());
  		}
  	}
  }
  	