#include <bits/stdc++.h>
using namespace std;
long long a[200000];
map<long long, bool> mm;
int main() {
  int n, t, k, d;
  cin >> n >> t >> k >> d;
  int res0 = 0, res1 = 0, sum0 = 0, sum1 = 0;
  while (sum0 < n) res0 += t, sum0 += k;
  for (int i = 1; i <= 10000; i++) {
    if (i % t == 0) sum1 += k, res1 = i;
    if (sum1 >= n) break;
    if (i % (d + t) == 0) sum1 += k, res1 = i;
    if (sum1 >= n) break;
  }
  if (res1 < res0)
    cout << "YES\n";
  else
    cout << "NO\n";
}
