Untitled

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
	int a, b, c, t,summ = 0;
	cin >> a >> b >> c >> t;
	for (int i = 1; i < t+1; i++)
	{
		if (i <= a) {
			summ += b;
		}
		else summ +=c;
	}
	cout << summ;
}