#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n, t, k, d, curr(0);
  cin >> n >> t >> k >> d;
  while (curr <= d) {
    n -= k;
    curr += t;
  }
  if (n > 0)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
