#include<cstdio>


int main(void)
{
	int i, n;
	double a[100+1]={4.0, 3.0};

	while(scanf("%d", &n)==1)
	{
		for(i=2;i<=n;i++)
		{
			a[i]=0.25*(19*a[i-1]-12*a[i-2]);
		}
		printf("%.30f\n", a[n]);
	}

	return 0;
}