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.*;
  
  public class Main {
      // For fast input output
      static class FastReader {
          BufferedReader br;
          StringTokenizer st;
  
          public FastReader() {
              try {
                  br = new BufferedReader(new FileReader("input.txt"));
                  PrintStream out = new PrintStream(new FileOutputStream("output.txt"));
                  System.setOut(out);
              } catch (Exception e) {
                  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;
          }
      }
      static  int gcd(int a,int b)
      {
          if(b==0)
          {
              return a;
          }
          return gcd(b,a%b);
      }
   
      static int lcm(int a, int b)
      {
          return (a / gcd(a, b)) * b;
      }
   
   
      static int abs(int a)
      {
          if(a<0)
              return -1*a;
          return a;
      }
      public static int lower(int arr[],int key){
          int low = 0;
          int high = arr.length-1;
          while(low < high){
              int mid = low + (high - low)/2;
              if(arr[mid] >= key){
                  high = mid;
              }
              else{
                  low = mid+1;
              }
          }
          return low;
      }
      public static int upper(int arr[],int key){
          int low = 0;
          int high = arr.length-1;
          while(low < high){
              int mid = low + (high - low+1)/2;
              if(arr[mid] <= key){
                  low = mid;
              }
              else{
                  high = mid-1;
              }
          }
          return low;
      }
  
      // end of fast i/o code
      public static void main(String[] args) {
          FastReader reader = new FastReader();
           StringBuilder ans = new StringBuilder();   
          int t = reader.nextInt();
          while (t-- > 0) {
              int c = reader.nextInt();
              int d = reader.nextInt();
  
              if(Math.abs(c-d)%2 == 1)
              ans.append("-1\n");
              else if( c == 0 && d == 0)
              ans.append("0\n");
              else if(c ==d)
              ans.append("1\n");
              else
              ans.append("2\n");
              
          }
          System.out.println(ans);
      }
      
    
  }