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)

use std::io::{Read};

fn main() -> Result<(), Box<dyn std::error::Error>> {
  let mut buf = String::new();
  std::io::stdin().read_to_string(&mut buf).unwrap();
  
  let mut lines = buf.lines();
  let n:usize = lines.next().unwrap().parse().unwrap();
  let s = lines.next().unwrap().trim().as_bytes();
  
  let mut rsum = vec![0; n + 1];
  for i in 1 ..= n {
    if s[i - 1] == b'R' {
      rsum[i] = rsum[i - 1] + 1;
    } else {
      rsum[i] = rsum[i - 1];
    }
  }

  let ans = (0 ..= n).map(|i|{
    let lw = i - rsum[i];
    let rr = rsum[n] - rsum[i];
    std::cmp::max(lw, rr)
  }).min().unwrap();
  
  println!("{}", ans);
  Ok(())
}