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.Generic;
using System.Linq;
using static System.Console;

public static class Program
{
    static char[] cArray;
    static int _firstWite;
    static int _lastRed;

    public static void Main()
    {
        int n = int.Parse(ReadLine());
        cArray = ReadLine().ToCharArray();
        int min = Math.Min(cArray.Count(x => x == 'R'), cArray.Count(x => x == 'W'));
        if(min == 0)
        {
            WriteLine(0);
            return;
        }
        {
            bool w = false;
            bool cmpl = true;
            for(int i = cArray.Length - 1; i >= 0; --i)
            {
                if (cArray[i] == 'W') w = true;
                else if (w)
                {
                    cmpl = false;
                    break;
                }
            }
            if (cmpl)
            {
                WriteLine(min);
                return;
            }
        }
        _firstWite = 0;
        _lastRed = cArray.Length - 1;
        int count = 0;
        while (!Check())
        {
            cArray[FirstWhite()] = 'R';
            cArray[LastRed()] = 'W';
            ++count;
            if(count >= min)
            {
                WriteLine(min);
                return;
            }
        }
        WriteLine(count);
    }

    static bool Check()
    {
        bool w = false;
        for(int i = _firstWite; i <= _lastRed; ++i)
        {
            if (cArray[i] == 'W') w = true;
            else if (w) return false;
        }
        return true;
    }

    static int FirstWhite()
    {
        for (int i = _firstWite; i < cArray.Length; ++i)
        {
            if (cArray[i] == 'W')
            {
                _firstWite = i;
                return i;
            }
        }
        _firstWite = -1;
        return -1;
    }

    static int LastRed()
    {
        for (int i = _lastRed; i >= 0; --i)
        {
            if (cArray[i] == 'R')
            {
                _lastRed = i;
                return i;
            }
        }
        _lastRed = -1;
        return -1;
    }
}