Decode the String

Deadshot , a secret undercover agent and a brilliant coder is on a secret mission to Chefland. He communicates with his officials using letters which contain a string $S$ and an integer key $K$. To decode the string $S$ he just takes every letter of the string and replaces it with its kth lexicographic descendant. For example if the letter is 'a' and $K=2$, 'a' will be replaced by 'c'. As Deadsho