#include <iostream>
#include <cstdio>
using namespace std;

int main()
{
	int a, b, m, n, cnt = 0, tmp1;
	cin >> a >> b >> m >> n;
	cnt += a + b;
	for (int i = 0; i < n; i++)
	{
		tmp1 = a + b;
		if (tmp1 <= m) cnt += tmp1; 
		else break;
		a = b; b = tmp1;
	}
	cout << cnt << endl;
	return 0;
}