#include <bits/stdc++.h>
using namespace std;
using ll = long long;
void solve(int tc = 0) {
  ll n, t, k, d;
  cin >> n >> t >> k >> d;
  ll res1, c = 0;
  for (ll i = 1;; i++) {
    if (i % t == 0) {
      c += k;
    }
    if (c >= n) {
      res1 = i;
      break;
    }
  }
  c = 0;
  ll res2;
  ll sec = 0;
  for (ll i = 1;; i++) {
    if (i == d) {
      sec = i;
    } else if (sec) {
      if ((i - sec) % t == 0) {
        c += k;
      }
    }
    if (i % t == 0) {
      c += k;
    }
    if (c >= n) {
      res2 = i;
      break;
    }
  }
  if (res1 > res2) {
    cout << "YES\n";
  } else {
    cout << "NO\n";
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  solve();
}
