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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.IO.Pipes;
using System.Linq;
using System.Numerics;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Text.RegularExpressions;
using System.Threading;

namespace Atcoder
{
	class Program
	{
		//string s = Console.ReadLine();
		//int[] intArray = Console.ReadLine().Split(' ').Select(i => int.Parse(i)).ToArray();
		//int H = intArray[0];
		//int W = intArray[1];
		//int N = int.Parse(Console.ReadLine());
		//List<int> A = new List<int>(Console.ReadLine().Split(' ').Select(i => int.Parse(i)).ToArray());
		//Boolean ok = false;
		//List<int> dp = Enumerable.Repeat(値, 個数).ToList();

		//Console.WriteLine(ans);
		//Console.WriteLine("Yes");
		//Console.WriteLine("No");
		//Console.ReadKey();
		//Console.WriteLine(cnt);

		//string[] strArray = Console.ReadLine().Split(' ').ToArray();
		//string S = strArray[0];
		//string T = strArray[1];
		static void Main(string[] args)
		{
			int N = int.Parse(Console.ReadLine());
			string CS = Console.ReadLine();
			int cnt = 0;
			if (CS.Contains("R") && CS.Contains("W"))
			{
				char[] C = CS.ToCharArray();
				while (true)
				{
					int Wind = CS.IndexOf("W");
					int Rind = CS.LastIndexOf("R");

					if (Wind < Rind)
					{
						C[Wind] = 'R';
						C[Rind] = 'W';
						CS = new string(C);
						cnt++;
					}
					else
					{
						break;
					}
				}
			}
			Console.WriteLine(cnt);
		}

		/// <summary>
		/// 回文
		/// </summary>
		/// <param name="Target"></param>
		/// <param name="len"></param>
		/// <returns></returns>
		static bool Palindrome(string Target , int len)
        {
			bool yes = true;

			for(int i = 0;i < len; i++)
            {
				if(Target[i] != Target[len - 1 - i])
                {
					yes = false;
					break;
                }
            }
			return yes;
        }

		/// <summary>
		/// アナグラム
		/// </summary>
		/// <param name="word1"></param>
		/// <param name="word2"></param>
		/// <returns></returns>
		static bool IsAnagramOf(string word1, string word2)
		{
			return word1.OrderBy(x => x).SequenceEqual(word2.OrderBy(x => x));
		}
		/// <summary>
		/// 最小公倍数
		/// </summary>
		/// <param name="a"></param>
		/// <param name="b"></param>
		/// <returns></returns>
		public static BigInteger Lcm(BigInteger a, BigInteger b)
		{
			return a * b / Gcd(a, b);
		}

		/// <summary>
		/// 最大公約数
		/// </summary>
		/// <param name="a"></param>
		/// <param name="b"></param>
		/// <returns></returns>
		public static BigInteger Gcd(BigInteger a, BigInteger b)
		{
			if (a < b)
				// 引数を入替えて自分を呼び出す
				return Gcd(b, a);
			while (b != 0)
			{
				var remainder = a % b;
				a = b;
				b = remainder;
			}
			return a;
		}
	}
}