#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, k, d, t1, t2, kol;
  bool f;
  f = false;
  cin >> n;
  cin >> t;
  cin >> k;
  cin >> d;
  t1 = 0;
  t2 = 0;
  kol = 0;
  while (kol < n) {
    t1++;
    if (t1 % t == 0) {
      kol = kol + k;
    }
  }
  kol = 0;
  while (kol < n) {
    t2++;
    if (f == true) {
      if ((t2 - d) % t == 0) {
        kol = kol + k;
      }
    }
    if (t2 % t == 0) {
      kol = kol + k;
    }
    if (t2 % d == 0) {
      f = true;
    }
  }
  if (t1 > t2) {
    cout << "YES";
  } else {
    cout << "NO";
  }
}
