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: 
  //package com.company;
  
  
  
  import  java.math.*;
  import java.util.*;
  import java.lang.*;
  import java.io.*;
  
  
  
  public final class Main {
  
      FastReader s;
  
  
      public static void main (String[] args) throws java.lang.Exception
      {
  
          new Main().run();
  
      }
      void run()
      {
          s = new FastReader();
          solve();
      }
      StringBuffer sb;
  
      void solve()
      {
  
          sb = new StringBuffer();
  
  
  
          for(int T = s.nextInt();T > 0;T--)
          {
  
              start();
  
          }
  
           System.out.print(sb);
  
      }
  
  
      void start()
      {
         int c = s.nextInt();
         int d = s.nextInt();
  
         if(c == 0 && d == 0)
         {
             sb.append("0\n");
             return;
         }
         if(Math.abs(c) == Math.abs(d))
         {
             sb.append("1\n");
                     return;
         }
  
         int a = Math.min(c,d);
          int b = Math.max(c,d);
        int diff = b-a;
          if(diff%2 == 1)
          {
              sb.append("-1\n");
              return;
          }
          sb.append("2\n");
      }
  
  
      int gcd(int a, int b)
      {
          if (b == 0)
              return a;
          return gcd(b, a % b);
      }
      long gcd(long a, long b)
      {
          if (b == 0)
              return a;
          return gcd(b, a % b);
      }
      long power(long x, long y, long p)
      {
          long res = 1; // Initialize result
  
          // Update x if it is more
          // than or equal to p
          x = x % p;
  
          while (y > 0)
          {
              // If y is odd, multiply
              // x with the result
              if ((y & 1) > 0)
                  res = (res * x) % p;
  
              // y must be even now
              y = y >> 1; // y = y/2
              x = (x * x) % p;
          }
          return res;
      }
  
      int lower_bound(int [] arr , int key)
      {
  
          int i = 0;
          int j = arr.length-1;
          //if(arr[i] > key)return -1;
          if(arr[j] < key)return -1;
  
          while(i<j)
          {
              int mid = (i+j)/2;
  
              if(arr[mid] == key)
              {
                  j = mid;
              }
  
              else if(arr[mid] < key)
              {
                  i = mid+1;
              }
              else
                  j = mid-1;
          }
  
          return i;
      }
  
      int upper_bound(int [] arr , int key)
      {
  
          int i = 0;
          int j = arr.length-1;
          if(arr[j] <= key)return j+1;
  
          while(i<j)
          {
              int mid = (i+j)/2;
  
  
  
              if(arr[mid] <= key)
              {
                  i = mid+1;
              }
              else
                  j = mid;
          }
  
          return i;
      }
  
      
  
      int find(int dsu [],int i)
      {
          if(dsu[i] == i)
              return i;
          dsu[i] = find(dsu,dsu[i]);
          return dsu[i];
      }
  
      void  union(int dsu[] ,int i, int j)
      {
          int a = find(dsu,i);
          int b = find(dsu,j);
          dsu[a] = b;
      }
  
      static void sort(int[] A){
          int n = A.length;
          Random rnd = new Random();
          for(int i=0; i<n; ++i){
              int tmp = A[i];
              int randomPos = i + rnd.nextInt(n-i);
              A[i] = A[randomPos];
              A[randomPos] = tmp;
          }
          Arrays.sort(A);
      }
      static void sort(long[] A){
          int n = A.length;
          Random rnd = new Random();
          for(int i=0; i<n; ++i){
              long tmp = A[i];
              int randomPos = i + rnd.nextInt(n-i);
              A[i] = A[randomPos];
              A[randomPos] = tmp;
          }
          Arrays.sort(A);
      }
      static String sort(String s) {
          Character ch[]=new Character[s.length()];
          for(int i=0;i<s.length();i++) {
              ch[i]=s.charAt(i);
          }
          Arrays.sort(ch);
          StringBuffer st=new StringBuffer("");
          for(int i=0;i<s.length();i++) {
              st.append(ch[i]);
          }
          return st.toString();
      }
  
      //long array input
      public long [] longArr(int len)
      {
          // long arr input
          long [] arr = new long[len];
          String [] strs =  s.nextLine().trim().split("\\s+");
          for(int i =0; i<len; i++)
          {
              arr[i] = Long.parseLong(strs[i]);
  
          }
          return arr;
      }
      // int arr input
      public int [] intArr(int len)
      {
          // long arr input
          int [] arr = new int[len];
          String [] strs =  s.nextLine().trim().split("\\s+");
          for(int i =0; i<len; i++)
          {
              arr[i] = Integer.parseInt(strs[i]);
  
          }
          return arr;
      }
  
      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());
          }
  
          String nextLine()
          {
              String str = "";
              try
              {
                  str = br.readLine();
              }
              catch (IOException e)
              {
                  e.printStackTrace();
              }
              return str;
          }
      }
  
  
  
  }
  