input: Write a function to generate the nth Fibonacci number.
  assert Fibonacci(1) == 0
  assert Fibonacci(2) == 1
  assert Fibonacci(9) == 21
  assert Fibonacci(10) == 34
output: def Fibonacci(n):
      if n == 1:
          return 0
      elif n == 2:
          return 1
      else:
          a, b = 0, 1
          for _ in range(n - 2):
              a, b = b, a + b
          return b