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(
  non_snake_case,
  unused_variables,
  unused_assignments,
  unused_mut,
  unused_imports,
  dead_code
)]
//use proconio::fastout;
use proconio::input;
use proconio::marker::*;
//use std::collections::HashSet;
// use std::cmp::*;
use std::collections::*;

fn main() {
  input! {
     N:usize,
     c:Chars
  }
  // 例 N=7
  //  W,W,W,W,W,W,R
  // ^ ^ ^ ^ ^ ^ ^ ^
  //         4
  let mut i = 0;
  let mut j = N;
  let mut ans = 0;
  while i < j {
    while i < N && c[i] == 'R' {
      i += 1;
    }
    while j > 0 && c[j - 1] == 'W' {
      j -= 1;
    }
    if i >= j {
      break;
    }
    ans += 1;
    i += 1;
    j -= 1;
  }
  println!("{}", ans);
}