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.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  import java.io.BufferedReader;
  import java.util.*;
  
  public class A {
  	private static final FastReader fs = new FastReader();
  	private static final PrintWriter out = new PrintWriter(System.out);
  	private static long m = 1_000_000_009;
  	private static long p = 31;
  	private static long mod = 1_000_000_007;
  	
  	
  	private static void solve() {
  		long c = fs.nextLong(), d = fs.nextLong();
  		long diff = abs(c-d);
  		if(diff % 2 != 0)
  			out.println(-1);
  		else {
  			if(c == d) {
  				if(c == 0)
  					out.println(0);
  				else
  					out.println(1);
  			}
  			else
  				out.println(2);
  		}
  	}
   
  	public static void main(String[] args) {
  		Thread t = new Thread(null, null, "", 1 << 28) {
  			public void run() {
  				int test_case = 1;
  				test_case = fs.nextInt();
  				for (int cs = 1; cs <= test_case; cs++)
  					solve();
  				out.close();
  			}
  		};
  		t.start();
  		try {
  			t.join();
  		} catch (InterruptedException e) {
  			e.printStackTrace();
  		}
  	}
   
  	private static class FastReader {
  		BufferedReader br;
  		StringTokenizer st;
   
  		public FastReader() {
  			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;
  		}
   
  		int[] readintarray(int n) {
  			int res[] = new int[n];
  			for (int i = 0; i < n; i++)
  				res[i] = nextInt();
  			return res;
  		}
   
  		long[] readlongarray(int n) {
  			long res[] = new long[n];
  			for (int i = 0; i < n; i++)
  				res[i] = nextLong();
  			return res;
  		}
  	}
  	
  	private static int max(int... arg) {
  		int length = arg.length;
  		if(length == 0)
  			throw null;
  		int maxVal = arg[0];
  		for(int i = 1; i < length; i++)
  			maxVal = Math.max(maxVal, arg[i]);
  		return maxVal;
  	}
  	
  	private static int min(int... arg) {
  		int length = arg.length;
  		if(length == 0)
  			throw null;
  		int maxVal = arg[0];
  		for(int i = 1; i < length; i++)
  			maxVal = Math.min(maxVal, arg[i]);
  		return maxVal;
  	}
  	
  	private static long max(long... arg) {
  		int length = arg.length;
  		if(length == 0)
  			throw null;
  		long maxVal = arg[0];
  		for(int i = 1; i < length; i++)
  			maxVal = Math.max(maxVal, arg[i]);
  		return maxVal;
  	}
  	
  	private static long min(long... arg) {
  		int length = arg.length;
  		if(length == 0)
  			throw null;
  		long maxVal = arg[0];
  		for(int i = 1; i < length; i++)
  			maxVal = Math.min(maxVal, arg[i]);
  		return maxVal;
  	}
  	
  	private static int abs(int a) {
  		return a >= 0 ? a : -a;
  	}
  	
  	private static int abs(int a, int b) {
  		return a-b >= 0 ? a-b : b-a;
  	}
  	
  	private static long abs(long a) {
  		return a >= 0 ? a : -a;
  	}
  	
  	private static long abs(long a, long b) {
  		return a-b >= 0 ? a-b : b-a;
  	}
  	
  	private static int power(int base, int exponent) {
  		int res = 1;
  		while(exponent > 0) {
  			if((exponent & 1) != 0)
  				res *= base;
  			exponent >>= 1;
  			base *= base;
  		}
  		return res;
  	}
  	
  	private static long power(long base, long exponent) {
  		long res = 1;
  		while(exponent > 0) {
  			if((exponent & 1) != 0)
  				res *= base;
  			exponent >>= 1;
  			base *= base;
  		}
  		return res;
  	}
  	
  	private static long mPower(long base, long exponent) throws Exception {
  		long res = 1;
  		base %= mod;
  		while(exponent > 0) {
  			if((exponent & 1) != 0)
  				res = mul(res, base);
  			exponent >>= 1;
  			base = mul(base, base);
  		}
  		return res;
  	}
  	
  	private static long mul(long a, long b) {
  		return a*b % mod;
  	}
  	
  	private static long add(long a, long b) {
  		return (a+b) % mod;
  	}
  	
  	private static long sub(long a, long b) {
  		long res = (a - b) % mod;
  		if(res < 0)
  			res += mod;
  		return res;
  	}
  }