What logic is involved in solving this?

Should I simply pick the last number in each list or should I pick the highest number in the list?

I am confused with the the first test case, where the sum ought to be 23. If I pick the last number in the list, the sum turns out to be 17 and not 23.

checkio([
        [1],
        [2, 3],
        [3, 3, 1],
        [3, 1, 5, 4],
        [3, 1, 3, 1, 3],
        [2, 2, 2, 2, 2, 2],
        [5, 6, 4, 5, 6, 4, 3]
    ])