#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int min = 0, min1 = 0, min2 = d, min3;
  int n1 = n;
  while (n1 > 0) {
    n1 -= k;
    min += t;
  }
  while (n > 0) {
    n -= k;
    min1 += t;
    if (min1 >= d && n > 0) {
      min2 += t;
      n -= k;
    }
  }
  min3 = (min1 > min2) ? min1 : min2;
  if (min <= min3)
    cout << "NO";
  else
    cout << "YES";
  return 0;
}
