P7767 [COCI 2011/2012 #5] DNA

There is a sequence of length $N$ consisting of the letters $A$ and $B$. Each operation can be one of the following two types: 1. Change one character in the sequence ($A\to B$ or $B\to A$). 2. Change a prefix of the sequence, i.e. perform operation 1 on characters $1$ through $K$ $(1\le K\le N)$. Find the minimum number of operations needed to make the entire sequence all $A$.

The first line contains an integer $N$. The second line contains $N$ characters, representing the sequence.

Output one line containing one integer, representing the answer.

$1\le N\le 10^{6}$. The sequence consists only of `'A'` and `'B'`. Translated from [COCI 2011/2012 #5 T3](https://hsin.hr/coci/archive/2011_2012/contest5_tasks.pdf). Translated by ChatGPT 5