Problem:
Score : 400 points

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

Solution in Rust (1.42.0) (judge verdict: Accepted)

use proconio::input;
#[allow(unused_imports)]
use proconio::marker::{Bytes, Chars};
#[allow(unused_imports)]
use std::cmp::{min, max};

fn main() {
	input! {
		_n: usize,
		mut cs: Chars,
	}

	let mut ans = 0;
	loop {
		// 先頭から続く赤を右方向に消していく
		loop {
			if 0 < cs.len() && cs[0] == 'R' {
				cs.remove(0);
			}
			else {
				break;
			}
		}
		if cs.len() == 0 {
			println!("{}", ans);
			return;
		}
		// 末尾から続く白を左方向に消していく
		loop {
			if 0 < cs.len() && cs[cs.len()-1] == 'W' {
				cs.remove(cs.len()-1);
			}
			else {
				break;
			}
		}
		if cs.len() == 0 {
			println!("{}", ans);
			return;
		}
		loop {
			// 先頭が白で末尾が赤ならそれぞれ削除して答えを＋＋
			if cs[0] == 'W' && cs[cs.len()-1] == 'R' {
				cs.remove(0);
				cs.remove(cs.len()-1);
				ans += 1;
				if cs.len() == 0 {
					println!("{}", ans);
					return;
				}
			}
			else {
				break;
			}
		}
	}
}