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.*;
  import java.text.*;
  
  public class CF_1556_A{
      //SOLUTION BEGIN
      void pre() throws Exception{}
      void solve(int TC) throws Exception{
          long A = nl(), B = nl();
          if((A+B)%2 == 1)pn(-1);
          else if(A == 0 && B == 0)pn(0);
          else if(A == B)pn(1);
          else pn(2);
      }
      //SOLUTION END
      void hold(boolean b)throws Exception{if(!b)throw new Exception("Hold right there, Sparky!");}
      void exit(boolean b){if(!b)System.exit(0);}
      static void dbg(Object... o){System.err.println(Arrays.deepToString(o));}
      final long IINF = (long)1e17;
      final int INF = (int)1e9+2;
      DecimalFormat df = new DecimalFormat("0.00000000000");
      double PI = 3.141592653589793238462643383279502884197169399, eps = 1e-8;
      static boolean multipleTC = true, memory = true, fileIO = false;
      FastReader in;PrintWriter out;
      void run() throws Exception{
          long ct = System.currentTimeMillis();
          if (fileIO) {
              in = new FastReader("");
              out = new PrintWriter("");
          } else {
              in = new FastReader();
              out = new PrintWriter(System.out);
          }
          //Solution Credits: Taranpreet Singh
          int T = multipleTC? ni():1;
          pre();
          for (int t = 1; t <= T; t++) solve(t);
          out.flush();
          out.close();
          System.err.println(System.currentTimeMillis() - ct);
      }
      public static void main(String[] args) throws Exception{
          if(memory)new Thread(null, new Runnable() {public void run(){try{new CF_1556_A().run();}catch(Exception e){e.printStackTrace();System.exit(1);}}}, "1", 1 << 28).start();
          else new CF_1556_A().run();
      }
      int[][] make(int n, int e, int[] from, int[] to, boolean f){
          int[][] g = new int[n][];int[]cnt = new int[n];
          for(int i = 0; i< e; i++){
              cnt[from[i]]++;
              if(f)cnt[to[i]]++;
          }
          for(int i = 0; i< n; i++)g[i] = new int[cnt[i]];
          for(int i = 0; i< e; i++){
              g[from[i]][--cnt[from[i]]] = to[i];
              if(f)g[to[i]][--cnt[to[i]]] = from[i];
          }
          return g;
      }
      int[][][] makeS(int n, int e, int[] from, int[] to, boolean f){
          int[][][] g = new int[n][][];int[]cnt = new int[n];
          for(int i = 0; i< e; i++){
              cnt[from[i]]++;
              if(f)cnt[to[i]]++;
          }
          for(int i = 0; i< n; i++)g[i] = new int[cnt[i]][];
          for(int i = 0; i< e; i++){
              g[from[i]][--cnt[from[i]]] = new int[]{to[i], i, 0};
              if(f)g[to[i]][--cnt[to[i]]] = new int[]{from[i], i, 1};
          }
          return g;
      }
      int find(int[] set, int u){return set[u] = (set[u] == u?u:find(set, set[u]));}
      int digit(long s){int ans = 0;while(s>0){s/=10;ans++;}return ans;}
      long gcd(long a, long b){return (b==0)?a:gcd(b,a%b);}
      int gcd(int a, int b){return (b==0)?a:gcd(b,a%b);}
      int bit(long n){return (n==0)?0:(1+bit(n&(n-1)));}
      void p(Object... o){for(Object oo:o)out.print(oo+" ");}
      void pn(Object... o){for(int i = 0; i< o.length; i++)out.print(o[i]+(i+1 < o.length?" ":"\n"));}
      void pni(Object... o){for(Object oo:o)out.print(oo+" ");out.println();out.flush();}
      String n()throws Exception{return in.next();}
      String nln()throws Exception{return in.nextLine();}
      int ni()throws Exception{return Integer.parseInt(in.next());}
      long nl()throws Exception{return Long.parseLong(in.next());}
      double nd()throws Exception{return Double.parseDouble(in.next());}
  
      class FastReader{
          BufferedReader br;
          StringTokenizer st;
          public FastReader(){
              br = new BufferedReader(new InputStreamReader(System.in));
          }
  
          public FastReader(String s) throws Exception{
              br = new BufferedReader(new FileReader(s));
          }
  
          String next() throws Exception{
              while (st == null || !st.hasMoreElements()){
                  try{
                      st = new StringTokenizer(br.readLine());
                  }catch (IOException  e){
                      throw new Exception(e.toString());
                  }
              }
              return st.nextToken();
          }
  
          String nextLine() throws Exception{
              String str;
              try{
                  str = br.readLine();
              }catch (IOException e){
                  throw new Exception(e.toString());
              }
              return str;
          }
      }
  }