input: Write a function in Python to identify the absent number in a sequence that is ordered.
  assert find_missing([1,2,3,5],4) == 4
  assert find_missing([1,3,4,5],4) == 2
  assert find_missing([1,2,3,5,6,7],5) == 4
output: def find_missing(sequence, n):
      return sum(range(1, n + 2)) - sum(sequence)