Untitled

#include <iostream>
using namespace std;
int main(){
	int a, b, c, t;
	cin >> a >> b >> c >> t;
	int s = 0;
	if (t <= a){
		cout << t * b;
	}
	else{
		s = a * b + (t - a) * c;
		cout << s;
	}
}