input: Write a Python function to locate the missing element in a sequence of numbers arranged in ascending order.
  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):
      expected_sum = n * (n + 1) // 2
      actual_sum = sum(sequence)
      return expected_sum - actual_sum