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