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 Main {
  	public static void main(String[] args) throws Exception {
  		FastIO io = new FastIO();
  		int test=io.nextInt();
  		while(test>0)
  		{
  		    long c=io.nextLong();
  		    long d=io.nextLong();
  		    if(c==d)
  		    {
  		        if(c==0)io.println(0);
  		        else io.println(1);
  		        test--;
  		        continue;
  		    }
  		    long diff=Math.abs(c-d);
  		    if(diff%2==1)io.println(-1);
  		    else
  		    {
  		        io.println(2);
  		    }
  		    test--;
  		}
  		io.close();
  	}
  }
   
   class FastIO extends PrintWriter {
  	private InputStream stream;
  	private byte[] buf = new byte[1<<16];
  	private int curChar, numChars;
   
  	// standard input
  	public FastIO() { this(System.in,System.out); }
  	public FastIO(InputStream i, OutputStream o) {
  		super(o);
  		stream = i;
  	}
  	// file input
  	public FastIO(String i, String o) throws IOException {
  		super(new FileWriter(o));
  		stream = new FileInputStream(i);
  	}
   
  	// throws InputMismatchException() if previously detected end of file
  	private int nextByte() {
  		if (numChars == -1) throw new InputMismatchException();
  		if (curChar >= numChars) {
  			curChar = 0;
  			try {
  				numChars = stream.read(buf);
  			} catch (IOException e) {
  				throw new InputMismatchException();
  			}
  			if (numChars == -1) return -1; // end of file
  		}
  		return buf[curChar++];
  	}
   
  	public String nextLine() {
  		int c; do { c = nextByte(); } while (c <= '\n');
  		StringBuilder res = new StringBuilder();
  		do { res.appendCodePoint(c); c = nextByte(); } while (c > '\n');
  		return res.toString();
  	}
   
  	public String next() {
  		int c; do { c = nextByte(); } while (c <= ' ');
  		StringBuilder res = new StringBuilder();
  		do { res.appendCodePoint(c); c = nextByte(); } while (c > ' ');
  		return res.toString();
  	}
   
  	public int nextInt() { 
  		int c; do { c = nextByte(); } while (c <= ' ');
  		int sgn = 1; if (c == '-') { sgn = -1; c = nextByte(); }
  		int res = 0;
  		do {
  			if (c < '0' || c > '9')
  				throw new InputMismatchException();
  			res = 10*res+c-'0';
  			c = nextByte();
  		} while (c > ' ');
  		return res * sgn;
  	}
   
    public long nextLong() { 
  		int c; do { c = nextByte(); } while (c <= ' ');
  		long sgn = 1; if (c == '-') { sgn = -1; c = nextByte(); }
  		long res = 0;
  		do {
  			if (c < '0' || c > '9')
  				throw new InputMismatchException();
  			res = 10*res+c-'0';
  			c = nextByte();
  		} while (c > ' ');
  		return res * sgn;
  	}
   
  	public double nextDouble() { return Double.parseDouble(next()); }
  }