input: def Fibonacci(n):
      """ Generate a Fibonacci sequence of first n numbers.
      >>> Fibonacci(1)
      0
      >>> Fibonacci(2)
      1
      >>> [Fibonacci(i) for i in range(1, 10)]
      [0, 1, 1, 2, 3, 5, 8, 13, 21]
      """
      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