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: Accepted)

object Main extends App {
  val inputs: Seq[String] = scala.io.Source.stdin.getLines().toList
  val n = inputs.head.toInt
  val stones = inputs(1)

  println(solve(n, stones))

  def solve(n: Int, stones: String): Int = {
    val rIndexes = scala.collection.mutable.ArrayBuffer.empty[Int]
    var ans = 0

    stones.zipWithIndex.foreach { case (c: Char, i: Int) =>
      if(c == 'R') {
        rIndexes.append(i)
      }
    }

    val rSize = rIndexes.size
    if (rSize == 0) {
      return 0
    }

    var rIndex = 0
    var rEnd = rIndexes.size - 1

    (0 until stones.length).foreach { i =>
      // 白は赤と交換
      if(stones(i) == 'W' && i < rIndexes(rEnd)) {
        ans = ans + 1
        rEnd = rEnd - 1
      }

      if (rEnd < 0) {
        return ans
      }
    }

    ans

  }
}