#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d, a = 0, b = 0;
  cin >> n >> t >> k >> d;
  int count = 0;
  while (count < n) {
    a += t;
    count += k;
  }
  count = 0;
  while (count < n) {
    b += 1;
    if (b % t == 0) {
      count += k;
    }
    if ((b - d) % t == 0 && b - d > 0) {
      count += k;
    }
  }
  if (b < a)
    cout << "YES";
  else
    cout << "NO";
}
