#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  int q = m * a;
  if (q < b) {
    cout << n * a;
  } else {
    int v = n / m;
    n -= v * m;
    cout << v * b + min(b, n * a);
  }
}
