# -*- coding: utf-8 -*-

N, = map(int, raw_input().split())

max2 = N / 2


fact = [1]
for i in xrange(1, 100):
    fact.append(i * fact[i-1])

total = 0
for i in xrange(max2+1):
    # 2をi回使う場合の数を求めて足す
    num1 = (N - 2 * i) # 1の回数
    add = fact[num1 + i] / fact[i] / fact[num1]
    total += add

print total