101743: [AtCoder]ABC174 D - Alter Altar

Problem Statement

An altar enshrines $N$ stones arranged in a row from left to right. The color of the $i$-th stone from the left $(1 \leq i \leq N)$ is given to you as a character $c_i$; R stands for red and W stands for white.

You can do the following two kinds of operations any number of times in any order:

• Choose two stones (not necessarily adjacent) and swap them.
• Choose one stone and change its color (from red to white and vice versa).

According to a fortune-teller, a white stone placed to the immediate left of a red stone will bring a disaster. At least how many operations are needed to reach a situation without such a white stone?

Constraints

• $2 \leq N \leq 200000$
• $c_i$ is R or W.

Input

Input is given from Standard Input in the following format:

$N$
$c_{1}c_{2}...c_{N}$

Output

Print an integer representing the minimum number of operations needed.

Sample Input 1

4
WWRR

Sample Output 1

2

For example, the two operations below will achieve the objective.

• Swap the $1$-st and $3$-rd stones from the left, resulting in RWWR.
• Change the color of the $4$-th stone from the left, resulting in RWWW.

Sample Input 2

2
RR

Sample Output 2

0

It can be the case that no operation is needed.

Sample Input 3

8
WRWWRWRR

Sample Output 3

3