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