import math
def combinations_count(n, r):
    return math.factorial(n) // (math.factorial(n - r) * math.factorial(r))

n = int(input())
ans = 0
for i in range(n):
    one = n-2*i
    two = i
    if one < 0:
        break
    ans += combinations_count(one+two, one)

print(ans)