#include <bits/stdc++.h>
using namespace std;
template <typename T>
T gcd(T a, T b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
template <typename T>
T pow(T a, T b, long long m) {
  T ans = 1;
  while (b > 0) {
    if (b % 2 == 1) ans = (ans * a) % m;
    b /= 2;
    a = (a * a) % m;
  }
  return ans % m;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long n, k, m, d;
  cin >> n >> k >> m >> d;
  long long ans = 0;
  for (long long i = 1; i <= d; i++) {
    long long tof = (n / (k * (i - 1) + 1));
    tof *= i;
    if (tof == 0) break;
    long long ftof = min(tof, m * i);
    ans = max(ans, ftof);
  }
  cout << ans;
}
