
Missing Number

You are given a sequence of integers, which are elements of arithmetic progression - the difference between the consecutive elements is constant. But this sequence is unsorted and one element is missing...good luck!
Input: List of integers (int).
Output: An integer (int).
Examples:
assert missing_number([1, 4, 2, 5]) == 3 assert missing_number([2, 6, 8]) == 4
Preconditions: