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.ArrayList;
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Map;
  import java.util.Scanner;
  import java.util.Set;
  import java.util.StringTokenizer;
  
  
  
  public class P01 {
  	  
  		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());
  			}
  			
  			float nextFloat() {
  				return Float.parseFloat(next());
  			}
  			String nextLine() {
  				String str = "";
  				try {
  					str = br.readLine();
  				}catch(IOException e) {
  					e.printStackTrace();
  				}
  				return str ;
  			}
  		}
  		
  		 public static int pascal(int n,int r){
  	           if(n==0||r==0||n==r){
  	             return 1;
  	            }else{
  	              return pascal(n-1,r-1)+pascal(n-1,r);
  	            }
  
  	     }
        
  		 public static int lower(int arr[],int key){
  			    int low = 0;
  			    int high = arr.length-1;
  			    while(low < high){
  			        int mid = low + (high - low)/2;
  			        if(arr[mid] >= key){
  			            high = mid;
  			        }
  			        else{
  			            low = mid+1;
  			        }
  			    }
  			    return low;
  			}
  	public static void main(String[] args) {
  	  FastReader fs = new FastReader();
  	  int t = fs.nextInt();
  	  while(t-->0) {
  		  long a = fs.nextLong();
  		  long b = fs.nextLong();
  		  if(Math.abs(b-a)%2==0) {
  			  if(a==b&&a==0) {
  				  System.out.println(0);
  			  }
  			  else if(a==b&&a>0) {
  				  System.out.println(1);
  			  }
  			  else if((a==0&&b>0)||(a>0&&b==0)) {
  				  System.out.println(2);
  			  }
  			  else {
  				 System.out.println(2);
  				  
  			  }
  		  }
  		  else {
  			  System.out.println(-1);
  		  }
  	  }     
  	}
  
  
  	private static int countdist(char[] ch,int x) {
  		// TODO Auto-generated method stub
  		int sum =0;
  		if(x==0) {
  			x=1;
  		}
  		if(x>1) {
  			x--;
  		}
  		for(int i =x;i<ch.length;i++) {
  			if(ch[i]==ch[i-1]) {
  				sum+=2;
  			}
  			else {
  				sum++;
  			}
  		}
  		return sum;
  	}
  
  
  	private static int reqs(int n) {
  		// TODO Auto-generated method stub
  		String s = "";
  		for(int i =0;i<Math.log10(n);i++) {
  			s+="1";
  		}
  		
  		return Integer.valueOf(s);
  	}
  	
  }
  
  