Codeforces_466A

// Codeforces 466A

#include <iostream>
#include <algorithm>

using namespace std;

int main()
{
    int n, m, a, b, s1 = 0, s2 = 0, s3 = 0, rez;
    cin >> n >> m >> a >> b;
    s1 = n*a;
    s2 = ((n/m)*b)+((n%m)*a);
    s3 = ((n/m)*b)+b;
    rez = min(s1, s2);
    rez = min(rez, s3);
    cout << rez;
    return 0;
}