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: Wrong Answer)

// -*- coding:utf-8-unix -*-

use proconio::input;
use proconio::marker::Chars;

fn main() {
    input! {
        n: u32,
        mut c: Chars,
    }

    // 尻から見る
    // 寄せられるだけ右に寄せて寄せきれなかったものは置換
    let mut ans = 0;
    let mut final_idx = c.len() as u32;
    c.reverse();
    // println!("{:?}", c);
    for i in 0..n - 2 {
        if i >= final_idx {
            break;
        }

        if c[i as usize] == 'R' {
            for j in (i + 1..n).rev() {
                if c[j as usize] == 'W' {
                    c[i as usize] = 'W';
                    c[j as usize] = 'R';
                    ans += 1;
                    for k in (i + 1..n).rev() {
                        if c[k as usize] == 'R' {
                            final_idx = k;
                        } else {
                            break;
                        }
                    }
                    break;
                }
            }

            if c[i as usize] == 'R' {
                c[i as usize] = 'W';
                ans += 1;
            }

            // println!("{:?}", c);
        }
    }
    println!("{}", ans);
}

// reversed
// rrwrwwrw
// wrwrwwrr
// wwwrwrrr
// wwwwrrrr

// WRRRRRW
// RRRRRRW