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 Scala (2.13.1) (judge verdict: Time Limit Exceeded)

import java.util.Scanner

import scala.util.control.Breaks

object Main {
  def main(args: Array[String]): Unit = {
    val sc = new Scanner(System.in)
    val n = sc.nextLine()
    val ss = sc.nextLine().split("").filter(_ != "")

    def rep(x: Int, y: Int): Unit = {
      val xx = ss(x)
      val yy = ss(y)
      ss(x) = yy
      ss(y) = xx
    }

    def check(wi: Int, ri: Int): Boolean = {
      if (wi < 0 || ri < 0) true
      else if (wi > ri) true
      else false
    }

    var cnt = 0
    val b = new Breaks
    b.breakable {
      while (true) {
        val lwi = ss.indexOf("W")
        val rri = ss.lastIndexOf("R")
        if (check(lwi, rri)) {
          b.break()
        } else {
          cnt += 1
          rep(lwi, rri)
        }
      }
    }

    println(cnt)

  }
}