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.math.*;
  import java.security.*;
  import java.text.*;
  import java.time.LocalTime;
  import java.util.*;
  import java.util.concurrent.*;
  import java.util.regex.*;
  
  
  public class A {
  	
  
  	public static void main(String[] args) {
      	FastScanner scan = new FastScanner();
      	PrintWriter out = new PrintWriter(System.out);
      	
      	int T = 1;
      	T = scan.nextInt();
      		for(int tt=0; tt<T; tt++) {
      			int c = scan.nextInt(), d = scan.nextInt();
      			int diff = Math.abs(c - d);
      			if(c == 0 && diff == 0) System.out.println(0);
      			else if(diff % 2 != 0) System.out.println(-1);
      			else {
      				System.out.println(diff == 0 ? 1 : 2);
      			}
      		}
  	}
  	
  	
  	
  	/*
  	static class IntComparator implements Comparator<Integer> {
  
  		public int compare(Integer o1, Integer o2) {
  		//	return Integer.compare(a[o1], a[o2]);
  			return 0;
  		}
  	}
  	*/
  	
  	
  	public static void printIntArray(int [] a) {
  		for(int i: a) System.out.print(i + " ");
  		System.out.println();
  	}
  	
  	public static void swapInt(int a, int b) {
  		int temp = a;
  		a = b;
  		b = temp;
  	}
  
  	
  	  public static void sort(int [] a) {
  	    	ArrayList<Integer> b = new ArrayList<>();
  	    	for(int i: a) b.add(i);
  	    	Collections.sort(b);
  	    	for(int i=0; i<a.length; i++) a[i]= b.get(i);
  	    }
  	  
  	  public static void reverse(int [] a) {
  	    	ArrayList<Integer> b = new ArrayList<>();
  	    	for(int i: a) b.add(i);
  	    	Collections.reverse(b);
  	    	for(int i=0; i<a.length; i++) a[i]= b.get(i);
  	    }
  
  
  		
  	
  	
      static class FastScanner{
      	
      	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      	StringTokenizer st = new StringTokenizer("");
      	String next() {
      		while(!st.hasMoreTokens())
      			try {
      				st = new StringTokenizer(br.readLine());
      			} catch (IOException e){
      				e.printStackTrace();
      			}
      		return st.nextToken();
      	}
      	
      	int nextInt() {
      		return Integer.parseInt(next());
      	}
      	
      	int [] readArray(int n) {
      		int [] a = new int[n];
      		for(int i=0; i<n ; i++) a[i] = nextInt();
      		return a;
      	}
      	
      	long nextLong() {
      		return Long.parseLong(next());
      	}
      	
      	
      }
      
      
     
  }
  
  