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 VOO {
  	public static void main(String[] args) {
  		MyScanner sc = new MyScanner();
  		out = new PrintWriter(new BufferedOutputStream(System.out));
  		int r = sc.nextInt();
  		for (int i = 0; i < r; i++) {
  			int c = sc.nextInt();
  			int d = sc.nextInt();
  
  			if (Math.abs((d + c)) % 2 == 1 || (c + d) / 2 < 0)
  				out.println(-1);
  			else if (d == 0 && c == 0)
  				out.println(0);
  			else if (d == c || d + c == 0)
  				out.println(1);
  			else
  				out.println(2);
  		}
  
  		// sc.nextInt();
  		// out.println(words);
  
  		// Stop writing your solution here. -------------------------------------
  		out.close();
  	}
  
  	// -----------PrintWriter for faster
  	// output---------------------------------printwriter for faster output,
  	// myscanner class for faster input. This is the way, this is why you are a
  	// disgrace to humanity and all it stands for you uncoordinated fuck
  	public static PrintWriter out;
  
  	// -----------MyScanner class for faster input----------
  	public static class MyScanner {
  		BufferedReader br;
  		StringTokenizer st;
  
  		public MyScanner() {
  			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());
  		}
  
  		String nextLine() {
  			String str = "";
  			try {
  				str = br.readLine();
  			} catch (IOException e) {
  				e.printStackTrace();
  			}
  			return str;
  		}
  
  	}
  	// --------------------------------------------------------
  }
  