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.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.OutputStreamWriter;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  
  public class A_A_Variety_of_Operations {
  
        static Scanner in=new Scanner();
      
    static PrintWriter out=new PrintWriter( new OutputStreamWriter(System.out) );
     
    static int testCases;
      
    static long c,d;
    
    static void solve(){
        
        if(c==d && c!=0){
            
            out.println(1);
            out.flush();
            
        }else if(c==d && c==0 ){
            
            out.println(0);
            out.flush();
            
        }else if( (c+d)%2==0 ){
            
            out.println(2);
            out.flush();
            
        }else{
            
            out.println(-1);
            out.flush();
            
        }
        
    }
      
      public static void main(String[] amit) throws IOException {
          
          testCases=in.nextInt();
          
          for(int t=0;t<testCases;t++){
              
              c=in.nextLong();
              
              d=in.nextLong();
              
              solve();
              
          }
          
          in.close();
          
      }
      
       static class Scanner{
          
          BufferedReader in;
          StringTokenizer st;
  
          public Scanner() {
          
              in=new BufferedReader( new InputStreamReader(System.in) );
              
          }
          
          String next() throws IOException{
              
              while(st==null || !st.hasMoreElements()){
                  
                  st=new StringTokenizer(in.readLine());
                  
              }
              
              return st.nextToken();
              
          }
          
          int nextInt() throws IOException{
              
              return Integer.parseInt(next());
              
          }
          
          long nextLong() throws IOException{
              
              return Long.parseLong(next());
              
          }
          
          String nextLine() throws IOException{
              
              return in.readLine();
              
          }
          
          char nextChar() throws IOException{
              
              return next().charAt(0);
              
          }
          
          double nextDouble() throws IOException{
              
              return Double.parseDouble(next());
              
          }
          
          float nextFloat() throws IOException{
              
              return Float.parseFloat(next());
              
          }
          
          boolean nextBoolean() throws IOException{
              
              return Boolean.parseBoolean(next());
              
          }
          
          void close() throws IOException{
              
              in.close();
              
          }
          
      }
      
  }