#include <bits/stdc++.h>



int main(){
  int A, B; std::cin >> A >> B;

  if(std::gcd(A, B) != 1){
    std::cout << -1 << std::endl;
  }else{
    int N = std::lcm(A, B);
    std::vector<int> check(N+1, 1);
    check[0] = 0;

    for(int x = 0; x <= B; ++x){
      for(int y = 0; y <= A; ++y){
        if(A*x+B*y <= N) check[A*x+B*y] = 0;
      }
    }

    int ans = std::count(check.begin(), check.end(), 1);

    std::cout << ans << std::endl;
  }

  return 0;
}