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