Prolonged Perishing

Chef has a string $S$ of length $N$ consisting of only lower case English alphabets (characters of the string are indexed from $0$ to $N-1$). He does the following in a single operation: 1) Finds a longest substring of $S$ such that all it’s characters are the same and deletes a single character from this substring (After deleting the $i$-th character from the string, the part of $S$ to the lef