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;

namespace test
{
    class MainClass
    {
        public static void Main(string[] args)
        {

            //問題162


            int n = int.Parse(Console.ReadLine());

            string cc = Console.ReadLine();

            char[] c = new char[n];

            char[] kansei = new char[n];


            int w_to_r = 0;
            int r_to_w = 0;

            for (int i = 0; i < n; i++)
            {
                c[i] = cc[i];

                if (c[i] == 'R')
                {
                    r_to_w += 1;
                }
                kansei[i] = 'W';


            }

            int ans = Math.Max(w_to_r, r_to_w);



            int count = 0;


            for (int i = 0; i < n; i++)
            {
                if (c[i] == 'W')
                {
                    w_to_r += 1;
                }
                else
                {
                    r_to_w -= 1;
                }

                //for (int j = 0; j < i; j++)
                //{
                //    kansei[j] = 'R';
                //}

                //Console.WriteLine("aaaaaaaa");


                //foreach (var a in kansei)
                //{
                //    Console.WriteLine(a);
                //}

                //foreach (var a in c)
                //{
                //    Console.WriteLine(a);
                //}

                //for (int j = 0; j < n; j++)
                //{
                //    if (c[j] != kansei[j])
                //    {
                //        count++;
                //        //Console.WriteLine("test");
                //    }
                //}

                //Console.WriteLine(count);

                count = Math.Max(w_to_r, r_to_w);

                //Console.WriteLine(count);


                //count = (count + 1) / 2;

                ans = Math.Min(ans, count);

                count = 0;
                //ans = n;

                
            }


            Console.WriteLine(ans);


        }

    }





    
}