Minimise Operations

You are given a String $S$ of length $N$. In one operation you can select any index $i$ of the string $(1 ≤ i ≤ N)$ and replace the character at index $i$ with any other character from the english alphabet. You can perform the operation any number of times. We define the successor of a character as its succeeding character from the english alphabet taken in a circular manner. For instance; $succ