input: Write a function to compute the n-th Fibonacci number.
  assert Fibonacci(5) == 3
  assert Fibonacci(10) == 34
  assert Fibonacci(1) == 0
output: def Fibonacci(n):
      if n <= 0:
          return 0
      elif n == 1:
          return 0
      elif n == 2:
          return 1
      
      a, b = 0, 1
      for _ in range(2, n):
          a, b = b, a + b
      return b