题解 | #兔子的数量#

n = int(input())
def digui(a):

    if a == 1 :
        return 2
    elif a == 2 :
        return 3
    else:
        while a >= 3 :
            return digui(a-1) + digui(a-2)

        a = a - 1
print(digui(n))