A Lost Cause

Pirate Keith loves cryptography and has protected his treasure with a very annoying caesar shift. He has witten “CGULKVIPFRGDOOCSJTRRVMORCQDZG” on his treasure chest and has left a piece of paper with the following message: “every subsequent letter is shifted one less than the previous.” Knowing this, can you unlock Pirate Keith’s treasure chest?

This challenge does not have any files to run, solve the challenge from above info and put the flag in the flagCheck binary to get the actual flag