input: def fibonacci_sequence(n: int) -> int:
      """Calculate the nth term in the Fibonacci sequence.
      >>> fibonacci_sequence(9)
      21
      >>> fibonacci_sequence(1)
      1
      >>> fibonacci_sequence(0)
      0
      >>> fibonacci_sequence(-1)
      "Incorrect input"
      """
output:     if n < 0:
          return "Incorrect input"
      if 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