input: from typing import List
  
  def Fibonacci(n: int) -> int:
      """Find the nth Fibonacci number.
      >>> Fibonacci(0)
      0
      >>> Fibonacci(1)
      1
      >>> Fibonacci(6)
      8
      >>> Fibonacci(10)
      55
      """
      if n < 0:
          raise ValueError("Incorrect input")
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return Fibonacci(n-1) + Fibonacci(n-2)
output:     if n < 0:
          raise ValueError("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