#include <bits/stdc++.h>
using namespace std;
mt19937_64 rang(
    chrono::high_resolution_clock::now().time_since_epoch().count());
int rng(int lim) {
  uniform_int_distribution<int> uid(0, lim - 1);
  return uid(rang);
}
const int mod = 1e9 + 7;
void solve() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int time = 0;
  while (time <= d) {
    time += t;
    n -= k;
  }
  if (n > 0)
    cout << "YES";
  else
    cout << "NO";
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
  srand(chrono::high_resolution_clock::now().time_since_epoch().count());
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
