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: Wrong Answer)

import java.util.*

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)
    val n = sc.nextInt()
    val c = sc.next()
    println(problem174d(n, c))
}

fun problem174d(n: Int, c: String): Int {
    var count = 0
    var foundW = 0
    for (i in 0 until n) {
        val ci = c[i]
        if (ci == 'W'&& foundW >= 0) {
            foundW++
        } else if (ci == 'R') {
            if (foundW >= 1) {
                count++
                foundW--
            }
        }
    }
    var noNeeds = 0
    for (i in 0 until n) {
        val ci = c[i]
        if (ci == 'R' && i + 1 < count) noNeeds++
    }
//    debugLog(count, noNeeds)
    return count - noNeeds
}