input: Write a python function to find the closest Fibonacci number less than a given positive integer.
  assert closest_fibonacci(10) == 8
  assert closest_fibonacci(15) == 13
  assert closest_fibonacci(1) == 0
  assert closest_fibonacci(22) == 21
output: def closest_fibonacci(n):
      if n <= 0:
          return 0
      a, b = 0, 1
      while b < n:
          a, b = b, a + b
      return a