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: Wrong Answer)

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

namespace contest_Csharp
{
    class Program
    {
        public static (long m,long s)[,] dp;

        static void Main(string[] args)
        {
            checked
            {
                var sw = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
                Console.SetOut(sw);

                var vals = Console.ReadLine().Split(' ').
                    Select(x => int.Parse(x)).ToList();

                var N = vals[0];
                var S = Console.ReadLine();
                var redseeker = N-1;
                var rcnt = S.Count(x=>x=='R');
                var ans = 0;
                for (int i = 0; i < N; i++)
                {
                    if (S[i] == 'W')
                    {
                        while (redseeker >= 0 && S[redseeker] != 'R')
                            redseeker--;
                        redseeker--;                   
                        rcnt--;
                        if (rcnt == 0 || redseeker <= i)
                            break;
                        ans++;
                        
                    }   
                    if (rcnt == 0 || redseeker <= i)
                            break;        
                }

                if (rcnt == S.Count(x => x == 'R'))
                    ans = 0;
                
                
                
                Console.WriteLine(ans);

                Console.Out.Flush();
            }




        }
    }
}