input: Write a python function to find the number in an array that is closest to a given number.
  assert closest_number([3, 5, 8, 11, 12], 4) == 3
  assert closest_number([1, 2, 3, 4, 5], 6) == 5
  assert closest_number([10, 20, 30, 40, 50], 25) == 20
  assert closest_number([-10, -20, -30, -40, -50], -25) == -20
  assert closest_number([7, 14, 21, 28, 35], 25) == 28
output: def closest_number(arr, target):
      return min(arr, key=lambda x: abs(x - target))