#include<fstream>
#include<iostream>
using namespace std;
int main()
{
    int a,d,r,f,v,c;
    cin>>a;
    d=a;
    cin>>a;
    r=d-a;
    c=a;
    cin>>a;
    f=a;
    if(d-a>r)
        v=d*a;
    cin>>a;
    v=+a*c;
    cout<<v;
    return 0;
}