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

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>

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

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

  int cnt = 0;
  for (int n = 1; n <= a*b-1; n++) {
    bool flag = false;
    rep(x, b+1) {
      if (n-a*x >= 0 && (n-a*x)%b == 0) flag = true;
    }
    if (flag) cnt++;
  }
  int ans = a*b-1 - cnt;
  cout << ans << endl;


}