input: def Fibonacci(n):
      """Generate a Fibonacci sequence of first n numbers.
      >>> Fibonacci(10)
      34
      >>> Fibonacci(5)
      3
      """
      if n==1:
          return 0
      elif n==2:
          return 1
      else:
          return Fibonacci(n-1)+Fibonacci(n-2)
output:     if n == 1:
          return 0
      elif n == 2:
          return 1
      else:
          a, b = 0, 1
          for _ in range(3, n + 1):
              a, b = b, a + b
          return b