#include <iostream>
using namespace std;
int main() {
int N, M, A, B;
cin>>N;
cin>>M;
cin>>A;
cin>>B;
int an,mb;
an=A*N;
for(int i=0; i<N; i+=M)
{
mb+=B;
}
if(an>=mb){
cout<<mb;}
else{
cout<<an;}
return 0;
}
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