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