Finding Maximal Non-Trivial Monotones

In this problem we will be dealing with character sequences, often called strings. A sequence is non-trivial if it contains at least two elements.

Given a sequence $s$, we say that a chunk $s\_i , \dots , s\_j$ is monotone if all its characters are equal, and we say that it is maximal if this chunk cannot be extended to left or right without losing the monotonicity.

Given a sequence composed only of characters “a” and “b”, determine how many characters “a” occur in non-trivial maximal monotone chunks.

The input consists of two lines. The first line contains a single integer $N$, where $1 ≤ N ≤ 10^5$. The second line contains a string with exactly $N$ characters, composed only of the characters “a” and “b”.

Print a single line containing an integer representing the total number of times the character “a” occurs in non-trivial maximal monotone chunks.