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: Runtime Error)

use proconio::{input, fastout};
use std::cmp::min;

#[fastout]
fn main() {
    input! {
        _n: i32,
        c: String,
    }

    let divide_pos = c.len()/2 + c.len()%2;
    let res =  solve(&c[0..divide_pos], &c[divide_pos..c.len()]);

    // println!("{:?}", res);
    println!("{}", min(res.0, res.1));

}

fn solve(lc: &str, rc: &str) -> (usize, usize) {
    // println!("{} {}", lc, rc);
    if lc.len() == 0 || rc.len() == 0 {
        return (0, 0)
    }

    let lc_w = lc.to_string().chars().filter(|&n| n.to_string().as_str() == "W").count();
    let rc_r = rc.to_string().chars().filter(|&n| n.to_string().as_str() == "R").count();

    if lc_w > 0 && rc_r > 0 {
        return (lc_w, rc_r);
    }

    if lc_w > 0 {
        let divide_pos = lc.len()/2 + lc.len()%2;
        return solve(&lc[0..divide_pos], &lc[divide_pos..lc.len()]);
    }

    if rc_r > 0 {
        let divide_pos = rc.len()/2 + rc.len()%2;
        return solve(&lc[0..divide_pos], &rc[divide_pos..rc.len()]);
    }

    panic!("UnExpected!!");
    (0, 0)
}