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: Time Limit Exceeded)

use std::io::*;
use std::str::FromStr;

pub fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

use std::cmp::{max, min};

fn main() {
    let n: usize = read();
    let mut s: Vec<char> = read::<String>().chars().collect();

    let mut last_w: i64 = -1;
    let mut last_r: i64 = -1;

    for i in 0..n {
        if s[i] == 'R' {
            last_r = i as i64;
        }
    }

    for i in (0..n).rev() {
        if s[i] == 'W' {
            last_w = i as i64
        }
    }

    if last_w == -1 || last_r == -1 {
        println!("0");
        return;
    }

    let mut head_w = s.len() - 1;
    let mut ans = 0;
    for i in 0..last_r as usize {
        if s[i] == 'W' {
            // println!("!!!, i:{}", i);
            let mut found = false;
            for j in (i..s.len()).rev() {
                // println!("j:{}", j);
                if s[j] == 'R' {
                    s[j] = 'W';
                    s[i] = 'R';
                    ans += 1;
                    found = true;
                    break;
                }
            }
        }
    }
    println!("{}", ans);
}