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)

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")
    };
}

fn main(){

    input!{
        n: usize,
        cs: chars,
    }

    // let mut cnt = 0;
    // let mut flag = false;
    // let mut r_end = 0;
    // for i in (0..n).rev() {
    //     if cs[i]=='R' {
    //         r_end = i;
    //         break;
    //     }
    //     // println!("{}", i);
    // }

    // for i in 0..n {
    //     if cs[i]=='W' {
    //         cnt += 1;
    //         for j in (0..r_end).rev() {
    //             if cs[j]=='R' {
    //                 r_end = j;
    //                 break;
    //             }
    //         }
    //         // if r_end<=i {
    //         //     break;
    //         // }
    //     }
    //     // println!("{}", r_end);
    // }
    // println!("{}", cnt);
    
    let mut ans: Vec<usize> = Vec::new();
    let mut r_cnt = 0;
    let mut w_cnt = 0;
    for i in 0..n+1 {
        for j in 0..n {
            if j<i {
                if cs[j]=='W' {
                    r_cnt += 1;
                }
            } else {
                if cs[j]=='R' {
                    w_cnt += 1;
                }
            }
        }
        ans.push(std::cmp::max(r_cnt, w_cnt));
        //println!("{}", std::cmp::max(r_cnt, w_cnt));
        r_cnt = 0;
        w_cnt = 0;
    }

    let mut min_ans = n;
    for i in ans {
        min_ans = std::cmp::min(min_ans, i);
    }
    println!("{}", min_ans);
}