HJ37
统计每个月兔子的总数
https://www.nowcoder.com/practice/1221ec77125d4370833fd3ad5ba72395
operation = int(input())
baby = 1
growing = 0
mature = 0
for i in range(0, operation-1):
mature += growing
growing = baby
baby = mature
print(baby + growing + mature)
