Pats send 2019 3rd for Shelton and a 2018 5th from CLE.
Makes sense for CLE, they aren't paying Shelton like a pass-rushing DT, and his 5th year option is gonna be 11 mill+. Good player, not worth the money. You can get run-stuffers.
They get a premium pick for Shelton, a player who would have walked with no compensation. (The Browns are some years away from getting comp picks, they are gonna be signing way more than they lose for two years, at least).
Pats on the other hand, get great run-stuffer, and will probably get a '20 comp pick (4th or 5th) from Shelton. So they gained a 5th this year, and will probably slide down a round or two from the '19 pick. Not terrible for them.