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)

// -*- coding:utf-8-unix -*-

use proconio::{fastout, input};

// #[fastout]
fn main() {
    input! {
        n: usize,
        c_str: String,
    }
    let c = c_str.as_bytes();
    let mut count = 0;
    let mut leftptr = 0;
    let mut rightptr = c.len() - 1;
    'mainloop: while leftptr < rightptr && leftptr < c.len() {
        while c[leftptr] == b'R' {
            leftptr += 1;
            if leftptr >= c.len() {
                break 'mainloop;
            }
        }
        while c[rightptr] == b'W' {
            if rightptr == 0 {
                break 'mainloop;
            }
            rightptr -= 1;
        }
        if leftptr >= rightptr {
            break;
        }
        count += 1;
        leftptr += 1;
        rightptr -= 1;
    }
    println!("{}", count);
}