#include <iostream>
#include <vector>
using namespace std;
int main(){
	vector<int> feb;
	int n, a, m, x;
	cin >> a >> n >> m >> x;
	feb.reserve(n);
	feb.push_back(1);feb.push_back(1);
	for(int i = 2; i < n; i++){
		feb[i] = feb[i - 1] + feb[i - 2];
	} 
	int b = (m - (feb[n - 4] + 1) * a) / (feb[n - 3] - 1);
	int out = (feb[x - 3] + 1) * a + (feb[x - 2] - 1) * b;
	cout << out << endl;
	return 0;
}
//5 7 32 4