#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int s;
  int t1, t2, t3;
  s = 0;
  t1 = t2 = t3 = 0;
  int t0 = INT_MAX;
  while (t3 < d) {
    s += k;
    t3 += t;
  }
  t2 = d;
  while (s < n) {
    s += k;
    t2 += t;
    if (s >= n) {
      t0 = t2;
      break;
    }
    s += k;
    t3 += t;
    if (s >= n) {
      t0 = t3;
      break;
    }
  }
  t1 = ceil(n / (1.0 * k)) * t;
  if (t0 >= t1)
    cout << "NO\n";
  else
    cout << "YES\n";
  return 0;
}
