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