#!/usr/bin/env python3
import fractions
n = int(input())
a = [None] * max(2, n + 1)
a[0] = 4
a[1] = 3
for i in range(2,len(a)):
    a[i] = (19 * a[i-1] -  12 * a[i-2]) / 4
print('{0:.10f}'.format(a[n]))