代码：

def f(n):
      f1,f2,f3=1,2,3
      if n<=3:
            return n
      else:
            for i in range(3,n):
                  fn=f3+f1
                  f1,f2,f3=f2,f3,fn
      return fn
while True:
      n=int(input())
      if n==0:
            break
      print(f(n))