#include <iostream>
using namespace std;

int main()
{
	int b;
	int f[21] = { 0 };
	int g[21] = { 0 };
	int a, n, m, x;
	cin >> a >> n >> m >> x;
	f[1] = a;
	int sum;
	for (int j = 0; j <= a; j++)
	{
		sum = a;
		b = j;
		f[2] = b;
		g[2] = b;
		for (int i = 3; i <= n - 1; i++)
		{
			f[i] = f[i - 2] + f[i - 1];
			g[i] = f[i - 1];
			sum += (f[i] - g[i]);
		}
		if (sum == m) break;
	}

	if (x == 1||x==2) cout << a << endl;
	else if(x < n)
	{
		sum = a;
		for (int i = 3; i <= x; i++)
		{
			f[i] = f[i - 2] + f[i - 1];
			g[i] = f[i - 1];
			sum += (f[i] - g[i]);
		}
		cout << sum << endl;
	}
	else if (x == n)
	{
		cout << 0 << endl;
	}

	return 0;
}

50分求助，前两个测试点都WA了