input: def Fibonacci(n): 
      if n<0: 
          return "Incorrect input"
      elif n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          return Fibonacci(n-1)+Fibonacci(n-2)
  
  """
  >>> Fibonacci(5)
  5
  >>> Fibonacci(10)
  55
  >>> Fibonacci(-1)
  "Incorrect input"
  """
output:     if n<0: 
          return "Incorrect input"
      elif n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b