while True: try: n = int(input()) a, b, c = 0, 0, 0 #a为出生1个月的兔子,b为出生2个月的兔子,c为出生2个月以上的兔子 if n == 1: #n为1~3时候特殊处理下 a = 1 elif n == 2: b = 1 elif n == 3: a = 1 c = 1 else: a = 1 c = 1 for i in range(4,n+1): #n为3以上时按照逻辑累加c即可 c += b b = a a = c print(a+b+c) except: break