A string $S$ of length $N$ is given. Each character of $S$ is either R or B. You can perform the following operation:

• Choose an index $i$: If the $i$-th character of $S$ is R, change it to B. If it is B, change it to R.

You want to perform given operations so that the set of positions where R appears is either empty or forms a single contiguous interval, and the set of positions where B appears is also either empty or forms a single contiguous interval. Find the minimum number of operations required.

The input is given in the following format.

$N$ $S$

Print the minimum number of operations required.

• $1 \le N \le 300\,000$.
• $S$ is a string of length $N$ consisting only of the characters R and B.