input: def fibonacci(n: int) -> int:
      """
      Calculate the nth number in the Fibonacci sequence.
      >>> fibonacci(9)
      21
      >>> fibonacci(0)
      0
      >>> fibonacci(-1)
      "Incorrect input"
      """
      a = 0
      b = 1
      if n < 0: 
          return "Incorrect input"
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      for i in range(2, n + 1): 
          c = a + b 
          a = b 
          b = c 
      return b
output:     a = 0
      b = 1
      if n < 0: 
          return "Incorrect input"
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      for i in range(2, n + 1): 
          c = a + b 
          a = b 
          b = c 
      return b