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.lang.*;
  import java.io.*;
  
  public class Main
  {
      static long mod = (int)1e9+7;
      public static void main (String[] args) throws java.lang.Exception
      {
          FastReader sc =new FastReader();
  
          int t=sc.nextInt();
     
       // int t=1;
     
          while(t-->0)
          {
              int c=sc.nextInt();
              int d=sc.nextInt();
              if(c==0&&d==0)
              {
                  System.out.println("0");
                  continue;
              }
              else if(c==d)
              {
                  System.out.println("1");
                  continue;
              }
              else if((int)Math.abs(c-d)%2!=0)
              {
                  System.out.println("-1");
                  continue;
              }
              else
              {
                  System.out.println("2");    
              }
          }
      }
  
  
  
  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();
          }
   
          int nextInt() { return Integer.parseInt(next()); }
   
          long nextLong() { return Long.parseLong(next()); }
   
          double nextDouble()
          {
              return Double.parseDouble(next());
          }
         
          float nextFloat()
          {
              return Float.parseFloat(next());
          }
   
          String nextLine()
          {
              String str = "";
              try {
                  str = br.readLine();
              }
              catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
         
          int[] readArray(int n) {
  int[] a=new int[n];
  for (int i=0; i<n; i++) a[i]=nextInt();
  return a;
  }
  
  long[] readArrayLong(int n) {
  long[] a=new long[n];
  for (int i=0; i<n; i++) a[i]=nextLong();
  return a;
  }
  
      }
     
      public static int[] radixSort2(int[] a)
  {
  int n = a.length;
  int[] c0 = new int[0x101];
  int[] c1 = new int[0x101];
  int[] c2 = new int[0x101];
  int[] c3 = new int[0x101];
  for(int v : a) {
  c0[(v&0xff)+1]++;
  c1[(v>>>8&0xff)+1]++;
  c2[(v>>>16&0xff)+1]++;
  c3[(v>>>24^0x80)+1]++;
  }
  for(int i = 0;i < 0xff;i++) {
  c0[i+1] += c0[i];
  c1[i+1] += c1[i];
  c2[i+1] += c2[i];
  c3[i+1] += c3[i];
  }
  int[] t = new int[n];
  for(int v : a)t[c0[v&0xff]++] = v;
  for(int v : t)a[c1[v>>>8&0xff]++] = v;
  for(int v : a)t[c2[v>>>16&0xff]++] = v;
  for(int v : t)a[c3[v>>>24^0x80]++] = v;
  return a;
  }
  
  
     
      public static HashMap<Integer, Integer> sortByValue(HashMap<Integer, Integer> hm)
      {
          // Create a list from elements of HashMap
          List<Map.Entry<Integer, Integer> > list =
                 new LinkedList<Map.Entry<Integer, Integer> >(hm.entrySet());
   
          // Sort the list
          Collections.sort(list, new Comparator<Map.Entry<Integer, Integer> >() {
              public int compare(Map.Entry<Integer, Integer> o1,
                                 Map.Entry<Integer, Integer> o2)
              {
                  return (o1.getValue()).compareTo(o2.getValue());
              }
          });
           
          // put data from sorted list to hashmap
          HashMap<Integer, Integer> temp = new LinkedHashMap<Integer, Integer>();
          for (Map.Entry<Integer, Integer> aa : list) {
              temp.put(aa.getKey(), aa.getValue());
          }
          return temp;
      }
  }