input: def fibonacci(n: int) -> int:
      """Calculate the nth number in the Fibonacci sequence.
      >>> fibonacci(0)
      0
      >>> fibonacci(1)
      1
      >>> fibonacci(9)
      21
      """
      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