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.*;
  
  /**
   * -----------------|___________|---------------------
   * CCCCCCCCC   OOOOOOOOOO   DDDDDDDD      EEEEEEEEE
   * CCCC        OOO    OOO   DD    DDD     EEEE
   * CCCC        OOO    OOO   DD     DDD    EEEEEEEEEE
   * CCCC        OOO    OOO   DD     DDDD   EEEE
   * CCCCCCCCC   OOOOOOOOOO   DDDDDDDDDDDD  EEEEEEEEEE
   * -----------------|___________|---------------------
   */
  
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          InputReader in = new InputReader(inputStream);
          PrintWriter sout = new PrintWriter(outputStream);
          int t = in.nextInt();
          while (t > 0) {
              t--;
              int c = in.nextInt();
              int d = in.nextInt();
              if(c==0 && d==0){
                  sout.println(0);
              }
              else if(c==d){
                  sout.println(1);
              }
              else if((c+d)%2==0){
                  sout.println(2);
              } else {
                  sout.println(-1);
              }
          }
          sout.close();
      }
  
      public static int gcd(int a, int b) {
          if (b == 0) {
              return a;
          }
          return gcd(b, a % b);
      }
  
      public static void sort(int[] a) {
          ArrayList<Integer> l = new ArrayList<>();
          for (int i : a)
              l.add(i);
          Collections.sort(l);
          for (int i = 0; i < a.length; i++)
              a[i] = l.get(i);
      }
  
      private static int upperBound(int[] a, int low, int high, int element) {
          while (low < high) {
              int middle = low + (high - low) / 2;
              if (a[middle] > element)
                  high = middle;
              else
                  low = middle + 1;
          }
          return low;
      }
  
      public static class InputReader {
          private BufferedReader reader;
          private StringTokenizer tokenizer;
  
          public InputReader(InputStream stream) {
              reader = new BufferedReader(new InputStreamReader(stream), 32768);
              tokenizer = null;
          }
  
          public String nextToken() {
              while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                  try {
                      tokenizer = new StringTokenizer(reader.readLine());
                  } catch (IOException e) {
                      throw new RuntimeException(e);
                  }
              }
              return tokenizer.nextToken();
          }
  
          public int nextInt() {
              return Integer.parseInt(nextToken());
          }
  
          public long nextLong() {
              return Long.parseLong(nextToken());
          }
  
          public double nextDouble() {
              return Double.parseDouble(nextToken());
          }
      }
  }
  