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::{fastout, input, marker::Bytes};
use std::cmp;

#[fastout]
fn main() {
    input! {
        _: usize,
        cs: Bytes,
    }

    let nr = cs.iter().filter(|&&c| c == b'R').count();
    let nw = cs.iter().filter(|&&c| c == b'W').count();

    let goal = itertools::repeat_n(b'R', nr).chain(itertools::repeat_n(b'W', nw));

    let mut s = 0;
    let mut t = 0;

    for (c1, c2) in itertools::zip(cs, goal) {
        match (c1, c2) {
            (b'W', b'R') => s += 1,
            (b'R', b'W') => t += 1,
            _ => {}
        }
    }

    let ans = s + t - cmp::min(s, t);
    println!("{}", ans);
}