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 text_io::*;
use std::collections::HashMap;
use std::process::exit;

fn main() {
    let n: usize = read!();
    let mut b:usize = 0;
    let g: &str = "R";
    let r: Vec<char> = g.chars().collect();
    let gg: &str = "W";
    let w: Vec<char> = gg.chars().collect();
    let a: String = read!();
    let v: Vec<char> = a.chars().collect();
    for i in 0..n {
        if v[i] == w[0] {
            b += 1;
        }
    }
    for i in 0..b {
        if v[i] == r[0] {
            b+=1;
        }
    }
    if b<=n/2 {
        println!("{}",b);
    }
    else {
        println!("{}",n-b);
    }
}