#include<stdio.h>

int main(void){
	double a[128]={0.0f};
	int i,n;
	a[0] = 4.0f;
	a[1] = 3.0f;
	for(i = 2;i <= 100;i++){
		a[i] = (19*a[i-1]- 12*a[i-2])/4;
	}
	scanf("%d",&n);
	printf("%.100lf",a[n]);
	return 0;
}