#include <bits/stdc++.h>
using namespace std;
const long double eps = 1e-9;
const int inf = (1 << 30) - 1;
const long long inf64 = ((long long)1 << 62) - 1;
const long double pi = acos(-1);
template <class T>
T sqr(T x) {
  return x * x;
}
template <class T>
T abs(T x) {
  return x < 0 ? -x : x;
}
int main() {
  ios_base::sync_with_stdio(0);
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  cout << ((d / t + 1) * k >= n ? "NO" : "YES") << endl;
  return 0;
}
