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.IO;
using System.Linq;

namespace Tasks
{
    public class D
    {
        static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
            Solve();
            Console.Out.Flush();
        }

        public static void Solve()
        {
            var N = int.Parse(Console.ReadLine());
            var C = Console.ReadLine().ToCharArray();
            var right = N - 1;
            var answer = 0;
            var R = C.Count(x => x == 'R');
            var W = N - R;
            if (R == 0 || R == N)
            {
                Console.WriteLine(0);
                return;
            }
            for (var i = R; i > 0; i--)
            {
                if (C[i] == 'R' && C[i - 1] == 'W')
                {
                    for (var j = right; j >= i; j--)
                    {
                        if (C[j] == 'R')
                        {
                            right = j - 1;
                            answer++;
                            (C[i - 1], C[j]) = (C[j], C[i - 1]);
                            break;
                        }
                    }
                }
            }
            // Console.WriteLine(new string(C));

            Console.WriteLine(answer);
        }
    }
}