cheap_travel.cpp

#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 << endl;
    else 
        cout << (n/m) * b + min((n%m) * a, b) << endl;
 
    return 0;
}