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: Wrong Answer)

use proconio::input;
use std::cmp::min;

fn main() {
    input! {
        _n: usize,
        cs: String
    }

    let mut cs: Vec<usize> = cs.chars().map(|c| if c == 'R' { 1 } else { 0 }).collect();

    let mut n1 = 0;
    while let Some(&e) = cs.first() {
        if e == 0 {
            cs.remove(0);
            loop {
                if cs.pop() != Some(0) {
                    break;
                }
            }
            n1 += 1;
        } else {
            break;
        }
    }

    let mut n2 = 0;
    if cs.len() > 0 {
        let r_sum: usize = cs[1..].iter().sum();
        n2 += min(r_sum, cs.len() - 1 - r_sum);
    }
    println!("{}", n1 + n2);
}