#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  if (m * a <= b)
    cout << n * a << "\n";
  else
    cout << (n / m) * b + min((n % m) * a, b) << "\n";
  return 0;
}
