input: Write a python function to determine the absent number in a sorted list.
  assert find_missing_number([1,2,3,5],4) == 4
  assert find_missing_number([1,3,4,5],4) == 2
  assert find_missing_number([1,2,3,5,6,7],5) == 4
output: def find_missing_number(arr, n):
      return (n * (n + 1) // 2) - sum(arr)