Chef and String Conversion

Given a string $S$ of length $N$. We can perform the following operation on the string as many times as we want :- Choose any index $i \ (1 \le i \le N)$ of the string. Let the character at this place be $a$. We can change the character $a$ to any character $b$ with the cost of the operation being $abs(ASCII(a) - ASCII(b))$. Here abs() indicates the absolute value function, which returns th