#include <iostream>
using namespace std;

int gcd(int a, int b) {
  int r = a % b;
  return r ? gcd(b, r) : b;
}

int main() {
  int a, b; cin >> a >> b;
  if (gcd(a, b) != 1) {
    cout << -1 << endl;
    return 0;
  }

  int m = (a - 1) * (b - 1);
  int ans = m + 1;
  for (int i = 0; i <= m/a; i++) {
    for (int j = 0; j <= m/b; j++) {
      if (a * i + b * j <= m) ans--;
    }
  }
  cout << ans << endl;
}