fibonacci ***Time limit exceeded***

[frequency]

def fib(number): 
  if number==0: 
      return 0
  elif number==1: 
      return 1
  else: 
    return fib(number-1)+fib(number-2)

n=int(input())
while n!="END" and n>0:
  print (fib(n))
  n=int(input())

With this code i can show the N fibonacci number given. But my code is not accepted because of this
***Time limit exceeded***. Any ideas?