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)

use proconio::{input, fastout};

#[fastout]
fn main() {
    input!(
        n: usize,
        c: String,
    );
    let mid = n / 2;
    let front = &c[..mid];
    let back = &c[mid..];
    
    let mut front_w_count: isize = 0;
    let mut back_r_count: isize = 0;

    if is_all_white(&c) || is_all_red(&c) { 
        println!("0");
        return;
    }

    for cf in front.chars() {
        if cf == 'W' { front_w_count += 1; }
    }
    for cb in back.chars() {
        if cb == 'R' { back_r_count += 1; }
    }

    println!("{}", front_w_count+(front_w_count-back_r_count).abs());
}

fn is_all_white(str: &String) -> bool {
    str.chars().all(|s| s == 'W' )
}

fn is_all_red(str: &String) -> bool {
    str.chars().all(|s| s == 'R')
}