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.lang.reflect.Array;
  import java.util.*;
  
  public class Main {
      // final CustomReader in = new CustomReader("input.txt");
      final CustomReader in = new CustomReader();
      final PrintWriter out = new PrintWriter(System.out);
  
      void solve() {
          int t = in.nextInt();
          while (t-- > 0) {
              int c = in.nextInt();
              int d = in.nextInt();
  
              if (c == 0 && d == 0) {
                  out.println(0);
                  continue;
              }
  
              if (Math.abs(c) == Math.abs(d)) {
                  out.println(1);
                  continue;
              }
  
              if ((c + d) % 2 == 1) {
                  out.println(-1);
                  continue;
              }
  
              out.println(2);
          }
      }
  
      public static void main(String[] args) { Main s = new Main(); s.solve(); s.out.close(); }
  
      private static class CustomReader {
          private final InputStream stream;
          private final byte[] buf = new byte[8192];
          private int curChar;
          private int numChars;
  
          CustomReader() {
              stream = System.in;
          }
  
          CustomReader(String fileName) {
              InputStream temp = null;
              try {
                  temp = new FileInputStream(fileName);
              } catch (FileNotFoundException e) {
                  e.printStackTrace();
              }
              stream = temp;
          }
  
          String nextLine() {
              int c = read();
              StringBuilder res = new StringBuilder();
              while (!isEndOfLine(c)) {
                  res.appendCodePoint(c);
                  c = read();
              }
              if (c == '\r') read();
              return res.toString();
          }
  
          String nextString() {
              int c = read();
              while (isSpaceChar(c)) c = read();
              StringBuilder res = new StringBuilder();
              while (!isSpaceChar(c)) {
                  res.appendCodePoint(c);
                  c = read();
              }
              if (c == '\r') read();
              return res.toString();
          }
  
          long nextLong() {
              int c = read();
              while (isSpaceChar(c)) c = read();
              int sgn = 1;
              if (c == '-') {
                  sgn = -1;
                  c = read();
              }
              long res = 0;
              while (!isSpaceChar(c)) {
                  if (c < '0' || c > '9') throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              }
              if (c == '\r') read();
              return res * sgn;
          }
  
          int nextInt() {
              int c = read();
              while (isSpaceChar(c)) c = read();
              int sgn = 1;
              if (c == '-') {
                  sgn = -1;
                  c = read();
              }
              int res = 0;
              while (!isSpaceChar(c)) {
                  if (c < '0' || c > '9') throw new InputMismatchException();
                  res *= 10;
                  res += c - '0';
                  c = read();
              }
              if (c == '\r') read();
              return res * sgn;
          }
  
          int[] nextArray(int n) {
              int[] arr = new int[n];
              for (int i = 0; i < n; i++) arr[i] = nextInt();
              return arr;
          }
  
          double nextDouble() {
              return Double.parseDouble(nextString());
          }
  
          boolean hasNext() {
              if (numChars == -1) return false;
              return curChar < numChars || curChar == buf.length || numChars == 0; // Probably
          }
  
          private int read() {
              if (numChars == -1) throw new InputMismatchException();
              if (curChar >= numChars) {
                  curChar = 0;
                  try {
                      numChars = stream.read(buf);
                  } catch (IOException e) {
                      throw new InputMismatchException();
                  }
                  if (numChars <= 0) return -1;
              }
              return buf[curChar++];
          }
  
          private static boolean isSpaceChar(int c) {
              return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
          }
  
          private static boolean isEndOfLine(int c) {
              return c == '\n' || c == '\r' || c == -1;
          }
      }
  }
  