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)

//【ライブラリここから】
// 1. 入力の容易化（https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8）
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}
//【ライブラリここまで】

fn main() {
    input! {
        n: i64,
        c_tmp: String,
    }
    let c = c_tmp.as_bytes();
    let mut wrc = Vec::<Vec<i64>>::with_capacity(n as usize);
    for _ in 0..n {
        let tmp_vec = vec![0, 0];
        wrc.push(tmp_vec);
    }
    for i in 0..n {
        let j = n - 1 - i;
        if i == 0 {
            wrc[i as usize][0] = 0;
            wrc[j as usize][1] = 0;
        } else {
            wrc[i as usize][0] = wrc[(i - 1) as usize][0];
            wrc[j as usize][1] = wrc[(j + 1) as usize][1];
        }
        if c[i as usize] == ('W' as u8) {
            wrc[i as usize][0] += 1;
        }
        if c[j as usize] == ('R' as u8) {
            wrc[j as usize][1] += 1;
        }
    }

    if wrc[(n - 1) as usize][0] < wrc[(n - 1) as usize][1] {
        println!("{}", 0);
    } else {
        for item in wrc {
            if item[0] >= item[1] {
                println!("{}", item[0]);
                break;
            }
        }
    }
}