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)

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

#[fastout]
fn main() {
    input!(
        n: usize,
        c: String,
    );

    let mut r2w = 0;
    let mut w2r = 0;

    for i in c.chars() {
        if i == 'R' { r2w += 1; }
    }

    let mut ans = cmp::max(r2w, w2r);

    for i in 0..n {
        if c.chars().nth(i).unwrap() == 'R' { r2w -= 1; }
        else { w2r += 1; }
        ans = cmp::min(ans, cmp::max(r2w, w2r));
    }

    println!("{}", ans);
}