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