n = int(input())

a0, a1 = 4., 3.

if n == 0:
    an = a0
elif n == 1:
    an = a0

for i in range(2, n+1):
    an = -3 * a0 + (19. / 4) * a1
    a1, a0 = an, a1
    if an < 10 ** -9:
        an = 0
        break

print(an)