Changing String

Fluffy the squirrel is playing with strings today. Today, he is given a string s consisting of lowercase letters. Every second, he can shift a letter to the left or right. For example, if the current letter is c, then he can change it to b or d in one second. Note that if we shift z to the right it becomes a and similarly a becomes z when shifted to the left. Now, Fluffy has k seconds to spare an