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.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.util.*;
  
  
  public class javacp{
  
  
      static FastReader fs=new FastReader();
      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;
          }
  
          int[] inputIntArray(int n){
              int[] arr = new int[n];
              for(int i=0;i<n;i++)
                  arr[i] = fs.nextInt();
  
              return arr;
          }
      }
  
      static void sort(int[] arr){
          ArrayList<Integer> al = new ArrayList<>();
          for(int val : arr){
              al.add(val);
          }
  
          Collections.sort(al);
          for(int i=0;i<arr.length;i++){
              arr[i] = al.get(i);
          }
      }
  
      static class Pair implements Comparable<Pair>{
          int vertex;
          long wt;
          int edgeno;
  
          Pair(int vertex , long wt , int edgeno){
              this.vertex = vertex;
              this.wt = wt;
              this.edgeno = edgeno;
          }
  
          public int compareTo(Pair pair){
              if(this.wt>pair.wt) return 1;
              else if(this.wt < pair.wt) return -1;
              else return this.edgeno-pair.edgeno;
          }
      }
  
  
      static int[] par1 , rank1;
      static int[] par2;
      static int[] rank2;
  
      static void union1(int vt1 , int vt2){
          int parent1 = findParent1(vt1) , parent2 = findParent1(vt2);
          int r1 = rank1[parent1] , r2 =  rank1[parent2];
  
          if(r1 < r2)
              par1[parent1] = parent2;
          else par1[parent2] = parent1;
  
          if(r1 == r2) rank1[parent1]++;
      }
  
      static void union2(int vt1 , int vt2){
          int parent1 = findParent2(vt1) , parent2 = findParent2(vt2);
          int r1 = rank2[parent1] , r2 =  rank2[parent2];
  
          if(r1 < r2)
              par2[parent1] = parent2;
          else par2[parent2] = parent1;
  
          if(r1 == r2) rank2[parent1]++;
      }
  
      static int findParent1(int vt){
          if(par1[vt] == vt) return vt;
          par1[vt] = findParent1(par1[vt]);
  
          return par1[vt];
      }
  
      static int findParent2(int vt){
          if(par2[vt] == vt) return vt;
          par2[vt] = findParent2(par2[vt]);
  
          return par2[vt];
      }
  
      static void initialise(int numOfVtces){
          par1 = new int[numOfVtces];
          par2 = new int[numOfVtces];
  
          rank1 = new int[numOfVtces];
          rank2 = new int[numOfVtces];
  
          for(int i=0;i<numOfVtces;i++){
              par1[i] = par2[i] = i;
              rank1[i] = rank2[i] = 0;
          }
      }
  
  
      public static void main(String[] args) throws IOException{
  
          int t = 1;
          t = fs.nextInt();
  
          while (t-- > 0) {
              int c = fs.nextInt() , d = fs.nextInt();
              if(c == d && d == 0) System.out.println(0);
              else if(c == d) System.out.println(1);
              else{
                  if(Math.abs(c-d) % 2 == 0)
                      System.out.println(2);
                  else System.out.println(-1);
              }
  
          }
      }
  
  
      static class CustomSort implements Comparator<int[]> {
  
          public int compare(int[] a, int[] b)
          {
              return a[0] - b[0];
          }
      }
  
      static void printArr(int[] arr){
          for(int i=0;i< arr.length;i++)
          {
              System.out.print(arr[i] + " ");
          }
          System.out.println();
      }
  
      private static int lcm(int a, int b) {
          int gcd = gcd(a,b);
          return (int)((long)a*(long)b)/gcd ;
      }
  
  
      private static int gcd(int a, int b) {
          if(b == 0) return a;
          else return gcd(b , a % b);
      }
  
  
      private static void swap(int[] arr, int start, int rand_pivot) {
          int temp = arr[start];
          arr[start] = arr[rand_pivot];
          arr[rand_pivot] = temp;
      }
  
  
  }