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 C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var n = int.Parse(Console.ReadLine());
        var c = Console.ReadLine();

        long ans = 0;

        // 後ろから貪欲的に
        int i = 0;
        int j = n - 1;

        while (i < c.Length && c[i] == 'R')
            i++;
        while (j >= 0 && c[j] == 'W')
            j--;

        while (true)
        {
            if (i >= j || i >= c.Length || j <= 0) break;

            ans++;
            i++;
            j--;

            while (i < c.Length && c[i] == 'R')
                i++;
            while (j >= 0 && c[j] == 'W')
                j--;
        }

        Console.WriteLine(ans);
    }

    public static bool isOK(char[] target)
    {
        if (target.All(t => t == 'R')) return true;
        if (target.All(t => t == 'W')) return true;

        bool isRed = true;
        for (int i = 0; i < target.Length; i++)
        {
            if (isRed)
            {
                if (target[i] == 'R') continue;
                else
                {
                    isRed = false;
                }
            }
            else
            {
                if (target[i] == 'W') continue;
            }
        }

        return true;
    }
}