#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline T abs(T t) {
  return t < 0 ? -t : t;
}
const long long modn = 1000000007;
inline long long mod(long long x) { return x % modn; }
const int MAXN = 212345;
int n, m, k;
int s[MAXN];
inline int sdasda(int a, int b) {
  if (a % b == 0) return a / b;
  return a / b + 1;
}
int fl(int a, int b) { return a / b; }
int main() {
  int t, d;
  scanf("%d %d %d %d", &n, &t, &k, &d);
  int res = sdasda(n, k) * t;
  int ans = INT_MAX;
  int cnt = 0;
  for (int a = 0; a <= 10000000; a++) {
    cnt = 0;
    if (a >= d) cnt += fl(a - d, t) * k;
    cnt += fl(a, t) * k;
    ;
    if (cnt >= n) {
      ans = a;
      break;
    }
  };
  if (res <= ans)
    puts("NO");
  else
    puts("YES");
}
