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.lang.*;
  import java.util.*;
  import java.io.*;
  
  public class Variety{
              static Integer[]strToIntArr(String s){
  	//	s = s.substring(1,s.length()-1);
  	//	String[] sArr = s.split(",");
  	String[] sArr = s.split(" ");	
  	int len = sArr.length;
  	Integer[] retArr = new Integer[len];
  	for(int i=0;i<len;i++){
  	    retArr[i] = Integer.parseInt(sArr[i]);
  	}
  	return retArr;
      }
  
      public int nOps(int c, int d){
  	int retV = -1;
  	int gap = Math.abs(c-d);
  	if((c==0)&&(d==0)){
  	    retV = 0;
  	}else if(c==d){
  	    retV = 1;
  	}else if((gap%2)==0){
  	    retV = 2;
  	}
  	return retV;
      }
      public static void main(String[]args) throws Exception{
  	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  	BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
  	String tLn = br.readLine();
  	Variety v = new Variety();
  	int t = Integer.parseInt(tLn);
  	for(int i=0;i<t;i++){
  	    String cdLn = br.readLine();
  	    Integer[]cdArr = strToIntArr(cdLn);
  	    int no = v.nOps(cdArr[0],cdArr[1]);
  	    bw.write(Integer.toString(no));
  	    bw.newLine();
  	    bw.flush();
  	}
  
      }
  }
  