#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, t, d, maxtime, cakes = 0, j;
  (scanf("%d%d%d%d", &n, &t, &k, &d));
  for (int i = 1;; i++) {
    if ((i % t) == 0) {
      cakes += k;
    }
    if (cakes >= n) {
      maxtime = i;
      break;
    }
  }
  cakes = 0;
  for (int i = 1;; i++) {
    if ((i % t) == 0) {
      cakes += k;
    }
    if ((i - d) % t == 0 && i > d) {
      cakes += k;
    }
    if (cakes >= n) {
      j = i;
      break;
    }
  }
  if (j < maxtime)
    cout << "YES";
  else
    cout << "NO";
}
