#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int c = 0, ans1, ans2;
  for (int i = 1;; i++) {
    if (i % t == 0) {
      c += k;
    }
    if (c >= n) {
      ans1 = i;
      break;
    }
  }
  c = 0;
  int sc = 0;
  for (int i2 = 1;; i2++) {
    if (i2 % t == 0) {
      c += k;
    }
    if (i2 == d) {
      sc = i2;
    } else if (sc) {
      if ((i2 - sc) % t == 0) {
        c += k;
      }
    }
    if (c >= n) {
      ans2 = i2;
      break;
    }
  }
  if (ans1 > ans2) {
    cout << "YES"
         << "\n";
  } else {
    cout << "NO"
         << "\n";
  }
}
