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 cp
  {
  	static class Reader {
  		final private int BUFFER_SIZE = 1 << 16;
  		private DataInputStream din;
  		private byte[] buffer;
  		private int bufferPointer, bytesRead;
  
  		public Reader()
  		{
  			din = new DataInputStream(System.in);
  			buffer = new byte[BUFFER_SIZE];
  			bufferPointer = bytesRead = 0;
  		}
  
  		public Reader(String file_name) throws IOException
  		{
  			din = new DataInputStream(
  					new FileInputStream(file_name));
  			buffer = new byte[BUFFER_SIZE];
  			bufferPointer = bytesRead = 0;
  		}
  
  		public String readLine() throws IOException
  		{
  			byte[] buf = new byte[64]; // line length
  			int cnt = 0, c;
  			while ((c = read()) != -1) {
  				if (c == '\n') {
  					if (cnt != 0) {
  						break;
  					}
  					else {
  						continue;
  					}
  				}
  				buf[cnt++] = (byte)c;
  			}
  			return new String(buf, 0, cnt);
  		}
  
  		public int nextInt() throws IOException
  		{
  			int ret = 0;
  			byte c = read();
  			while (c <= ' ') {
  				c = read();
  			}
  			boolean neg = (c == '-');
  			if (neg)
  				c = read();
  			do {
  				ret = ret * 10 + c - '0';
  			} while ((c = read()) >= '0' && c <= '9');
  
  			if (neg)
  				return -ret;
  			return ret;
  		}
  
  		public long nextLong() throws IOException
  		{
  			long ret = 0;
  			byte c = read();
  			while (c <= ' ')
  				c = read();
  			boolean neg = (c == '-');
  			if (neg)
  				c = read();
  			do {
  				ret = ret * 10 + c - '0';
  			} while ((c = read()) >= '0' && c <= '9');
  			if (neg)
  				return -ret;
  			return ret;
  		}
  
  		public double nextDouble() throws IOException
  		{
  			double ret = 0, div = 1;
  			byte c = read();
  			while (c <= ' ')
  				c = read();
  			boolean neg = (c == '-');
  			if (neg)
  				c = read();
  
  			do {
  				ret = ret * 10 + c - '0';
  			} while ((c = read()) >= '0' && c <= '9');
  
  			if (c == '.') {
  				while ((c = read()) >= '0' && c <= '9') {
  					ret += (c - '0') / (div *= 10);
  				}
  			}
  
  			if (neg)
  				return -ret;
  			return ret;
  		}
  
  		private void fillBuffer() throws IOException
  		{
  			bytesRead = din.read(buffer, bufferPointer = 0,
  					BUFFER_SIZE);
  			if (bytesRead == -1)
  				buffer[0] = -1;
  		}
  
  		private byte read() throws IOException
  		{
  			if (bufferPointer == bytesRead)
  				fillBuffer();
  			return buffer[bufferPointer++];
  		}
  
  		public void close() throws IOException
  		{
  			if (din == null)
  				return;
  			din.close();
  		}
  	}
  
  	public static boolean check(int arr[],int sort[]) {
  		for(int i = 0;i < arr.length;i++) {
  			if(arr[i] != sort[i]) {
  				return true;
  			}
  
  		}
  		return false;
  	}
  
  	
  
  
  	public static void main(String[] args) throws IOException
  	{
  		try{
  			//Your Solve
  			Reader s = new Reader();
  //			Scanner s = new Scanner(System.in);
  			
  			int t = s.nextInt();
  			for(int p = 0;p < t;p++) {
  				int c = s.nextInt();
  				int d = s.nextInt();
  				if(c==d && c==0) {
  					System.out.println(0);
  				}else if(c==d){
  					System.out.println(1);
  				}else if((Math.abs(c-d)%2)==0){
  					System.out.println(2);
  				}else {
  					System.out.println(-1);
  				}
  				
  				
  			}	
  
  		}catch(Exception e){
  			return;
  		}
  	}
  }