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.util.*;
  import java.io.*;
  import java.text.DecimalFormat;
  	public class problem{
  		 static Scanner sc = new Scanner(System.in); 
  		 static   PrintWriter pWriter= new PrintWriter(System.out);
  		 static LinkedList<Integer> list = new LinkedList<Integer>();
  			static class Scanner {
  				StringTokenizer st;
  				BufferedReader br;
  		 
  				public Scanner(InputStream s) {
  					br = new BufferedReader(new InputStreamReader(s));
  				}
  		 
  				public Scanner(FileReader r) {
  					br = new BufferedReader(r);
  				}
  		 
  				public String next() throws IOException {
  					while (st == null || !st.hasMoreTokens())
  						st = new StringTokenizer(br.readLine());
  					return st.nextToken();
  				}
  		 
  				public int nextInt() throws IOException {
  					return Integer.parseInt(next());
  				}
  		 
  				public long nextLong() throws IOException {
  					return Long.parseLong(next());
  				}
  		 
  				public String nextLine() throws IOException {
  					return br.readLine();
  				}
  		 
  				public double nextDouble() throws IOException {
  					String x = next();
  					StringBuilder sb = new StringBuilder("0");
  					double res = 0, f = 1;
  					boolean dec = false, neg = false;
  					int start = 0;
  					if (x.charAt(0) == '-') {
  						neg = true;
  						start++;
  					}
  					for (int i = start; i < x.length(); i++)
  						if (x.charAt(i) == '.') {
  							res = Long.parseLong(sb.toString());
  							sb = new StringBuilder("0");
  							dec = true;
  						} else {
  							sb.append(x.charAt(i));
  							if (dec)
  								f *= 10;
  						}
  					res += Long.parseLong(sb.toString()) / f;
  					return res * (neg ? -1 : 1);
  				}
  		 
  				public long[] nextlongArray(int n) throws IOException {
  					long[] a = new long[n];
  					for (int i = 0; i < n; i++)
  						a[i] = nextLong();
  					return a;
  				}
  		 
  				public Long[] nextLongArray(int n) throws IOException {
  					Long[] a = new Long[n];
  					for (int i = 0; i < n; i++)
  						a[i] = nextLong();
  					return a;
  				}
  		 
  				public int[] nextIntArray(int n) throws IOException {
  					int[] a = new int[n];
  					for (int i = 0; i < n; i++)
  						a[i] = nextInt();
  					return a;
  				}
  		 
  				public Integer[] nextIntegerArray(int n) throws IOException {
  					Integer[] a = new Integer[n];
  					for (int i = 0; i < n; i++)
  						a[i] = nextInt();
  					return a;
  				}
  		 
  				public boolean ready() throws IOException {
  					return br.ready();
  				}
  		 
  			}
  			public static void display(char[][]a) {
  				for(int i=0;i<a.length;i++) {
  					for(int j=0;j<a[i].length;j++) {
  						System.out.print(a[i][j]);
  					}
  					System.out.println();
  					
  					
  			}
  			}
  			public static void main(String args[] ) throws IOException{
  			int n=sc.nextInt();
  			while(n-->0) {
  				int c = sc.nextInt();
  				int d = sc.nextInt();
  				if(c==0&&d==0) pWriter.println(0);
  				else if(Math.abs(d-c)%2!=0) pWriter.println(-1);
  				else if(c==d) pWriter.println(1);
  				else pWriter.println(2);
  			
  			}
  pWriter.flush();
  			}
  			public static boolean primeCheck(int n) {
  				for(int i=2;i<n;i++) {
  					if(n%i==0) {
  						pWriter.println(i);
  						return true;
  					}
  				}
  				return false;
  			}
  }
  	
  