题解 | #兔子的数量#

num = int(input())


def fac(n):
    res = [2, 3]
    if n == 1:
        return 2
    if n == 2:
        return 3

    for i in range(3, n + 1):
        res.append(res[i - 2] + res[i - 3])
    return res[n - 1]


print(fac(num))