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: Time Limit Exceeded)

use whiteread::parse_line;

fn main() {
    let n: usize = parse_line().unwrap();
    let s: String = parse_line().unwrap();
    let mut cs: Vec<_> = s.chars().collect();
    let mut cnt = 0;
    for i in 0..n {
        for j in (i..n).rev() {
            let cond = cs[i] == 'W' && cs[j] == 'R';
            if cond {
                let tmp = cs[i];
                cs[i] = cs[j];
                cs[j] = tmp;
                cnt += 1;
                break;
            }
        }
    }
    // println!("{:?}", cs);
    println!("{}", cnt)
}