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)

fn d() {
    input! {
        n: u32,
        mut c: Chars
    }

    let mut left = 0;
    let mut right = n - 1;
    let mut ans = 0;

    loop {
        for l in left..n {
            if c[l as usize] == 'W' {
                left = l;
                break;
            }
        }
        for r in (0..right).rev() {
            if c[r as usize] == 'R' {
                right = r;
                break;
            }
        }
        if left >= right {
            break;
        }
        c[left as usize] = 'R';
        c[right as usize] = 'W';
        left += 1;
        right -= 1;
        ans += 1;
    }

    for i in 0..(n - 1) {
        let s = i as usize;
        if c[s] == 'W' && c[s + 1] == 'R' {
            ans += 1;
        }
    }
    println!("{}", ans);
}

use proconio::{input, marker:: Chars};

fn main() {
    d();
}