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: Accepted)

use proconio::input;
// result = R...RW...W
fn main() {
    input! {
        n: usize,
        c: String,
    }
    let c: Vec<_> = c.chars().collect();
    let (mut lw, mut rr) = (vec![0; n + 1], vec![0; n + 1]);
    rr[0] = c.iter().filter(|&x| x == &'R').count();
    for i in 0..n {
        rr[i + 1] = rr[i];
        lw[i + 1] = lw[i];
        match c[i] {
            'R' => rr[i + 1] -= 1,
            'W' => lw[i + 1] += 1,
            _ => unreachable!(),
        }
    }

    println!(
        "{:?}",
        (0..=n)
            .map(|i| {
                let swap = std::cmp::min(lw[i], rr[i]);
                let change_r = std::cmp::max(rr[i] - swap, 0);
                let change_l = std::cmp::max(lw[i] - swap, 0);
                swap + change_r + change_l
            })
            .min()
            .unwrap()
    );
}