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 Kotlin (1.3.71) (judge verdict: Accepted)

import kotlin.math.min

fun main() = Thread(null, ::abc174d, "(^o^)", 128 * 1024 * 1024).apply { start() }.join()

@OptIn(ExperimentalStdlibApi::class)
fun abc174d() {
    val n = readLine()!!.toInt()
    val line = readLine()!!

    val firstR = line.indexOf("R")
    val lastW = line.indexOf("W")

    val wCount = line.count { it == 'W' }
    val rCount = line.count { it == 'R' }
    val answer = if (wCount == 0 || rCount == 0) 0 else {
        minOf(wCount, rCount, min(
            line.slice(0 until rCount).count { it == 'W' },
            line.slice((line.lastIndex - wCount + 1)..line.lastIndex).count { it == 'R' }
        ))
    }

    println(answer)
}