#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
const int N = 1010100;
bool dp[N];
signed main() { 
  ios::sync_with_stdio(false); cin.tie(0);
  int a, b;
  cin >> a >> b;
  if (__gcd(a, b) != 1) {
    cout << -1 << endl;
    return 0;
  }
  dp[0] = true;
  for (int i = 1; i < N; i++) {
    if (i - a >= 0 && dp[i - a]) dp[i] = true;
  }
  for (int i = 0; i < N; i++) {
    if (i - b >= 0 && dp[i - b]) dp[i] = true;
  }
  int ans = 0;
  for (int i = 1; i < N; i++) {
    ans += (int) !dp[i];
  }
  cout << ans << endl;
  return 0;
}