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};
use proconio::marker::Chars;

#[fastout]
fn main() {
    input! {
        n: usize,
      	mut stones: Chars,
    }

    let cnt_W = stones.iter().filter(|&n| n == &'W').count() as usize;
  	let border = n - cnt_W;
    
    let mut iter = stones.iter().enumerate();
    let mut W_pos = std::vec::Vec::with_capacity(cnt_W);
    loop {
        let mut ret = iter.next();
        match ret {
            Some((i, val)) => {
                if val == &'W' && i < border{
                    W_pos.push(i);
                }
            },
            _ => break,
        }
    }

    println!("{}", W_pos.iter().count());
    /* for pos in W_pos {
        let R_pos = stones.iter().rposition(|&x| x == &'R');
        stones.swap(pos, R_pos);
    } */

}