The program reads the two header entries, then the $n$ processing times, keeping the largest seen, and writes it. The loop costs $15$ per entry and the whole command $12$ more, and the compiler charges $10$ machine instructions per unit of that cost plus a final $halt$, so $151 · (|x| + 1)$ covers it.

The domain is the submission's standard fitting condition rather than a bound on the sum of the word: the value bound the IMP+ program needs is the length of the word plus its largest entry, which one entry's fitting condition already supplies.