def find_missing_number(C, size: int) -> int:
    """Find the missing number.

    Find the missing number using the
    difference between sum of known and
    arithmetic sums.
    """
    known_numbers_sum = sum(C)
    expected_numbers_sum = (size/2) * (C[0] + C[-1])

    return int(expected_numbers_sum - known_numbers_sum)