input: def fibonacci(n: int) -> int: 
      """
      Calculate the nth Fibonacci number in the sequence.
      >>> fibonacci(9)
      34
      >>> fibonacci(1)
      1
      >>> fibonacci(0)
      0
      """
      a = 0
      b = 1
      if n < 0: 
          return "Incorrect input" 
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          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 
      else: 
          for i in range(2, n + 1): 
              c = a + b 
              a = b 
              b = c 
          return b