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)

#[allow(unused_imports)]
use proconio::{
    fastout, input,
    marker::{Bytes, Chars, Isize1, Usize1},
};

fn main() {
    input! {
        n:usize,
        c:Chars,
    }
    let r = c.iter().filter(|x| **x == 'R').count();
    let w = n - r;
    let ans = std::cmp::min(r, w);
    let mut p1 = 0;
    let mut p2 = n - 1;
    let mut buf = 0;
    if n == 2 {
        if c[0] == 'W' && c[1] == 'R' {
            println!("1");
        } else {
            println!("0");
        }
        return;
    }
    while p1 < p2 {
        while p1 < p2 && c[p1] == 'R' {
            p1 += 1;
        }
        while p1 < p2 && c[p2] == 'W' {
            p2 -= 1;
        }
        if p1 < p2 {
            buf += 1;
            p1 += 1;
            p2 -= 1;
        }
    }
    println!("{}", std::cmp::min(ans, buf));
}