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: // Generated by Code Flattener.
  // https://plugins.jetbrains.com/plugin/9979-idea-code-flattener
  
  import java.io.*;
  import java.util.StringTokenizer;
  
  public class Main {
      public static void main(String[] args) {
          Solution solution = new ASolution();
  
          boolean local = System.getProperty("ONLINE_JUDGE") == null && !solution.isInteractive();
          String input = local ? "input.txt" : null;
  
          solution.in = new MyScanner(input);
          solution.out = MyWriter.of(null);
          solution.bm = new Benchmark();
          solution.al = new Algorithm();
          solution.run();
          solution.close();
      }
  
      private abstract static class Solution {
          public MyScanner in;
          public MyWriter out;
          public Benchmark bm;
          public Algorithm al;
  
          public void init() {
          }
  
          public abstract void solve();
  
          protected boolean isMultiTest() {
              return true;
          }
  
          public void run() {
              init();
              if (isMultiTest()) {
                  int t = in.nextInt();
                  for (int i = 0; i < t; i++) {
                      solve();
                  }
              } else {
                  solve();
              }
          }
  
          public void close() {
              out.close();
          }
  
          public boolean isInteractive() {
              return false;
          }
      }
  
      private static class ASolution extends Solution {
          public void solve() {
              int c = in.nextInt();
              int d = in.nextInt();
              if (c == 0 && d == 0) {
                  out.println(0);
              } else if (Math.abs(c) == Math.abs(d)) {
                  out.println(1);
              } else if ((c - d) % 2 == 0) {
                  out.println(2);
              } else {
                  out.println(-1);
              }
          }
      }
  
      private static class MyScanner {
          private final BufferedReader br;
          private StringTokenizer st;
  
          public MyScanner(String fileName) {
              if (fileName != null) {
                  try {
                      File file = new File(getClass().getClassLoader().getResource(fileName).getFile());
                      br = new BufferedReader(new FileReader(file));
                  } catch (FileNotFoundException e) {
                      throw new RuntimeException(e);
                  }
              } else {
                  br = new BufferedReader(new InputStreamReader(System.in));
              }
          }
  
          public String next() {
              while (st == null || !st.hasMoreElements()) {
                  st = new StringTokenizer(nextLine());
              }
              return st.nextToken();
          }
  
          public int nextInt() {
              return Integer.parseInt(next());
          }
  
          public String nextLine() {
              try {
                  String line = br.readLine();
                  if (line == null) {
                      throw new RuntimeException("empty line");
                  }
                  st = null;
                  return line;
              } catch (IOException e) {
                  throw new RuntimeException(e);
              }
          }
      }
  
      private static class Benchmark {
      }
  
      private static class Algorithm {
      }
  
      private static class MyWriter extends PrintWriter {
          public static MyWriter of(String fileName) {
              if (fileName != null) {
                  try {
                      return new MyWriter(new FileWriter(fileName));
                  } catch (IOException e) {
                      throw new RuntimeException(e);
                  }
              } else {
                  return new MyWriter(new BufferedOutputStream(System.out));
              }
          }
  
          public MyWriter(FileWriter fileWriter) {
              super(fileWriter);
          }
  
          public MyWriter(OutputStream out) {
              super(out);
          }
      }
  }