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 proconio::marker::Chars;

fn main() {
    input!(n: usize);
    input!(mut c: Chars);
    let mut result = 0;
    let mut stones: Vec<String> = Vec::new();

    for i in 0..n {
        // println!("stones: {}", stones.join(""));
        if stones.is_empty() {
            stones.push(c.get(i).unwrap().to_string());
            continue;
        }
        if c.get(i).unwrap().to_string() == String::from("W") {
            stones.push(c.get(i).unwrap().to_string());
        } else {
            if stones.get(i - 1).unwrap().to_string() == String::from("W") {
                if i == n - 1 {
                    stones.push("W".to_string());
                    result += 1;
                } else {
                    stones.push(c.get(i).unwrap().to_string());
                    stones.swap(0, i);
                    result += 1;
                }
            } else {
                stones.push(c.get(i).unwrap().to_string());
            }
        }
    }
    // println!("stones: {}", stones.join(""));
    println!("{}", result);
}