题解 | #统计每个月兔子的总数#

n = int(input())
x = 1
arr = [
    x, # 1
    0, # 2
    0, # 3+
]

# print(arr)

for i in range(n-1):
    nt = arr[2] + arr[1]
    arr[2] += arr[1]
    arr[1] = arr[0]
    arr[0] = nt
    # print(arr)


print(sum(arr))