#include <bits/stdc++.h>
using namespace std;
mt19937 gen(time(NULL));
const long double eps = 1e-9;
const int inf = 1e9;
const int mod = 1e9 + 7;
const long long infinity = 2 * 1e18;
int n, t, k, d;
int w[100005];
int main() {
  cin >> n >> t >> k >> d;
  int st = 0;
  for (int i = 0; i < n; i += k) st += t;
  int dt = 0;
  dt += d;
  int sum = 0;
  for (int time = 0; time + t <= d; time += t) sum += k;
  dt = d;
  for (; sum < n; sum += 2 * k) dt += t;
  if (dt < st)
    puts("YES");
  else
    puts("NO");
}
