#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
int n,k,x,y,ans;

signed main() {
	cin>>n>>k>>x>>y;
	if(n<=k){
		ans=n*x;
	}else{
		n-=k;
		ans=k*x+n*y;
	}
	cout<<ans<<"\n";
	return 0;
}//4 9 10