#include <bits/stdc++.h>
using namespace std;

int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  int a, b;
  cin >> a >> b;
  if (__gcd(a, b) == 1) {
    bitset<10010> bs;
    bs[0] = 1;
    for (int i = 0; i < 10010; ++i) {
      if (i + a < 10010 and bs[i]) {
        bs[i + a] = 1;
      }
      if (i + b < 10010 and bs[i]) {
        bs[i + b] = 1;
      }
    }
    int res = 0;
    for (int i = 0; i < 10010; ++i) {
      res += bs[i] == 0;
    }
    cout << res << '\n';
  } else {
    cout << "-1\n";
  }
}