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.util.Arrays;
  
  public class AVarietyOfOperations {
      private class Solution {
  
          private Solution() {
  
          }
  
          private int solve(int[] A) {
              if (A[0] == A[1]) return A[0] == 0 ? 0 : 1;
              if (((A[0] + A[1]) & 1) == 1) return -1;
              else return 2;
          }
      }
  
      private static class InputUtils {
          private static BufferedReader BR;
          private static InputStreamReader inputStreamReader;
  
          public static BufferedReader getBR() {
              if (null == BR) {
                  inputStreamReader = new InputStreamReader(System.in);
                  BR = new BufferedReader(inputStreamReader);
              }
              return BR;
          }
  
          public static int[] nextLineIntArray() {
              return Arrays.stream(splitNextLine()).mapToInt(Integer::valueOf).toArray();
          }
  
          public static Integer[] nextLineIntegerArray() {
              return Arrays.stream(splitNextLine()).mapToInt(Integer::valueOf).boxed().toArray(Integer[]::new);
          }
  
          public static String[] splitNextLine() {
              return splitNextLine(BR, "\\s");
          }
  
          public static String[] splitNextLine(BufferedReader br) {
              return splitNextLine(br, " ");
          }
  
          public static String[] splitNextLine(BufferedReader br, String regex) {
              return nextLine(br).split(regex);
          }
  
          public static String nextLine() {
              return nextLine(getBR());
          }
  
          public static String nextLine(BufferedReader br) {
              try {
                  return br.readLine();
              } catch (IOException e) {
                  return null;
              }
          }
  
          public static int toInteger(String s) {
              return Integer.valueOf(s);
          }
  
          public static long toLong(String s) {
              return Long.valueOf(s);
          }
  
          public static int nextInt() {
              return toInteger(nextLine());
          }
  
          public static long nextLong() {
              return toLong(nextLine());
          }
      }
  
      private static class OutputUtils {
          public static void print(Object o) {
              System.out.print(o);
          }
  
          public static void println(Object o) {
              System.out.println(o);
          }
      }
  
      private static void print(Object o) {
          OutputUtils.print(o);
      }
  
      private static void println(Object o) {
          OutputUtils.println(o);
      }
  
      public static void main(String[] args) {
          int t = InputUtils.nextInt();
          var object = new AVarietyOfOperations();
          while (t-- > 0) {
               var line = InputUtils.nextLineIntArray();
               println(object.new Solution().solve(line));
          }
      }
  }
  
  