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)

#![allow(non_snake_case)]
#![allow(unused_imports)]
#![allow(dead_code)]

use proconio::{input, fastout};
use proconio::marker::*;
use whiteread::parse_line;
use std::collections::*;
use num::*;
use num_traits::*;
use superslice::*;
use std::ops::*;

use itertools::Itertools;
use itertools_num::ItertoolsNum;


#[fastout]
fn solve() {
    const MOD: usize = 1_000_000_007;
    const INF: usize = std::usize::MAX;
    
    input!{
        n: usize,
        c_vec: Chars,
    }

    if c_vec.iter().all_equal() {
        println!("0")
    } else {
        let mut r_pos = VecDeque::new();
        let mut w_pos = VecDeque::new();
        for i in 0..n {
            if c_vec[i] == 'R' {
                r_pos.push_back(i);
            } else {
                w_pos.push_back(i);
            }
        }
        // dbg!(&r_pos, &w_pos);

        let mut ans = 0;
        loop {
            let r_last = r_pos.pop_back().unwrap();
            let w_first = w_pos.pop_front().unwrap();

            if r_last < w_first {
                break;
            } else {
                r_pos.push_front(w_first);
                w_pos.push_back(r_last);
            }
            ans += 1;
        }

        println!("{}", ans)
    }
    println!("", );
}

fn main() {
    solve()
}

#[cfg(test)]
mod test {
    use super::solve;
}