//
// main.cpp
// A. Cheap Travel
//
// Created by Farrah Tharwat on 08/02/2023.
//
#include <iostream>
#define endl '\n'
using namespace std;
int main() {
int n,m,a,b,money=0;
cin>>n>>m>>a>>b;
while(n){
if(a*m >= b && n >= m){
n-=m;
money+=b;
}
else{
n--;
money+=a;
}
}
cout<<money<<endl;
}
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