#include <bits/stdc++.h>
using namespace std;
int n, t, k, d;
int main() {
  scanf("%d%d%d%d", &n, &t, &k, &d);
  int tm1 = n / k;
  if (tm1 * k < n) tm1++;
  tm1 *= t;
  int tm2 = 0;
  int ns1 = 0, ns2 = 0;
  int ns = 0;
  for (int i = 1; i <= tm1; i++) {
    ns1++;
    if (i > d) ns2++;
    if (ns1 >= t) ns += k, ns1 -= t;
    if (ns2 >= t && i > d) ns2 -= t, ns += k;
    if (i != tm1 && ns >= n) {
      puts("YES\n");
      return 0;
    }
  }
  puts("NO\n");
  return 0;
}
