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 test {
  
      static long mod = 1000000007;
      static long inf = Long.MAX_VALUE;
  
      public static void solve(long c, long d) {
          if(c == 0 && d == 0) {
              System.out.println(0);
              return;
          }
          if(c == d) {
              System.out.println(1);
              return;
          }
          if((Math.abs(c-d) % 2) == 1) {
              System.out.println(-1);
              return;
          }
          System.out.println(2);
      }
      
  /*
  input:
  6
  1 2
  3 5
  5 3
  6 6
  8 0
  0 0
  
  
  output:
  -1
  2
  2
  1
  2
  0
  
  
  */
  
      public static void main(String[] args) {
          FastReader sc = new FastReader();
          int t = sc.nextInt();
          while(t-- > 0) {
              long c = sc.nextLong();
              long d = sc.nextLong();
              // solve
              solve(c, d);
          }
      }
  
       
  	public static void sort(long[] a) {
  		ArrayList<Long> l=new ArrayList<>();
  		for (long i:a) l.add(i);
  		Collections.sort(l);
  		for (int i=0; i<a.length; i++) a[i]=l.get(i);
  	}
  
      public static int maxIndex(long[] arr) {
          long max = Long.MIN_VALUE;
          int index = 0;
          int n = arr.length;
          for(int i = 0; i < n; i++) {
              if(arr[i] > max) {
                  index = i;
                  max = arr[i];
              }
          }
          return index;
      }
  
      public static int minIndex(long[] arr) {
          long min = Long.MAX_VALUE;
          int index = 0;
          int n = arr.length;
          for(int i = 0; i < n; i++) {
              if(arr[i] < min) {
                  index = i;
                  min = arr[i];
              }
          }
          return index;
      }
  
      public static void debugArray(long[] arr) {
          for(int i = 0; i < arr.length; i++) {
              System.out.print(arr[i] + " ");
          }
          System.out.println();
      } 
  
      public static void debugArrayList(ArrayList<Long> arr) {
          for(Long i : arr) {
              System.out.print(i + " ");
          }
          System.out.println();
      }
  
      public static long gcd(long a, long b) {
          if(b == 0) {
              return a;
          }
          else {
              return gcd(b, a % b);
          }
      }
   
      public static long LCM(long a, long b) {
          return Math.abs(a*b) / gcd(a, b);
      }
  
      static class FastReader {
          BufferedReader br;
          StringTokenizer st;
      
          public FastReader() {
              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();
          }
  
          String nextLine() {
              String str="";
              try {
                  str=br.readLine();
              }
              catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
  
          double nextDouble() {
              return Double.parseDouble(next());
          }
  
          long[] readArray(int n) {
              long[] a = new long[n];
              for(int i = 0; i < n ; i++) {
                  a[i] = nextLong();
              }
              return a;
          }
      }
  }