#include<iostream>
#include<algorithm>
#include<vector>
#include<functional>
#include<string>
#include<cmath>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<cstdio>
#include<numeric>
using namespace std;
using ll = long long;
int main(){
  ll a, b;
  cin >> a >> b;
  if(a != -1 && b != 0){
    cout << -1 << endl;
  }else{
    if(b == 0){
      cout << 1 << endl;
    }else if(a == -1){
      cout << 2 << endl;
    }
  }
  return 0;
}