#include<stdio.h>
int main()
{
	int a, n, m, x;
	int b = 1;
	int i;
	scanf_s("%d %d %d %d", &a, &n, &m, &x);
	//a为首发人数，n为车站数目；m为终点站人数。x为所求
	int up[20];//每站上车人数
	int down[20];//每站下车人数
	int sum[20];//每站人数
	sum[n - 1] = -1;
	up[0] = a;up[1] = b;
	down[0] = 0;down[1] = up[1];
	sum[0] = a;
	while (1) {
for (i = 0;i < n;i++)
	{
		sum[i + 1] = sum[i] + up[i + 1] - down[i + 1];
		down[i + 2] = up[i+1];
		up[i + 2] = up[i] + up[i + 1];
	}
if (sum[n - 2] != m)
		{
	up[1] = up[1] + 1;
			down[1] = up[1];
			
		}
		else
		{
			printf("%d", sum[x-1]);
			return 0;
			break;

		}
	}
	
}