#include <iostream>
#include <cmath>
using namespace std;
int main(){
    int a,b,c,x;
    cin>>a>>b>>c>>x;
    //A店铺
    int priceA;
    if(x>=b){
        priceA=a*x;
    }else{//CAUTION!这种情况也可以！
        priceA=a*b;
    }
    
    //B店铺
    int priceB;
    priceB=c*x;
    
    //价格比较
    int price=priceA>priceB?priceB:priceA;
    cout<<price<<endl;
    return 0;
}