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;
use proconio::marker::Chars;

#[allow(dead_code)]
fn dump(a: &Vec<bool>) -> String {
    a.iter().map(|&c| if c { 'W' } else { 'R' } ).collect()
}

#[allow(dead_code)]
fn type_of<T>(_: T) -> &'static str {
    std::any::type_name::<T>()
}

fn main() {
    input!{
        _: usize,
        a: Chars,
    }
    let mut a = a.iter().map(|&c| c == 'W').collect::<Vec<bool>>();

    let mut itr = a.iter_mut();
    let mut ans = 0usize;
    while let Some(c) = itr.by_ref().skip_while(|c| !**c).next() {
        if let Some(s) = itr.rfind(|s| !**s) {
            std::mem::swap(c, s);
            ans += 1;
        }
    }
    println!("{}", ans);


}