import math

N = int(input())

two_chance = N // 2
#print(two_chance)
ans = 0

for i in range(0, two_chance + 1):
    #print(math.factorial(N - i) / (math.factorial(i) * math.factorial(N - (2 * i))))
    ans += math.factorial(N - i) / (math.factorial(i) * math.factorial(N - (2 * i)))

print(int(ans))