input: Write a python function to find the missing number in an arithmetic sequence.
  assert find_missing_number([1, 2, 4, 5]) == 3
  assert find_missing_number([10, 11, 13, 14]) == 12
  assert find_missing_number([100, 101, 102, 104]) == 103
output: def find_missing_number(sequence):
      n = len(sequence) + 1
      total_sum = n * (sequence[0] + sequence[-1]) // 2
      actual_sum = sum(sequence)
      return total_sum - actual_sum