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;

namespace AtCoderBeginnerContest174
{
	class Program
	{
		static void Main(string[] args)
		{
			var input = int.Parse(Console.ReadLine());
			var colors = Console.ReadLine().ToCharArray();

			var count = 0;
			while (!Check(colors))
			{
				count++;
				var white = GetIndex(colors, 'W');
				var red = GetIndex(colors, 'R');

				colors[white] = 'R';
				colors[red] = 'W';
			}

			Console.WriteLine(count);
		}

		static bool Check(Char[] colors)
		{
			var white = GetFirstIndex(colors, 'W');
			var red = GetLastIndex(colors, 'R');

			if (red > white)
			{
				return false;
			}
			else
			{
				return true;
			}
		}

		static int GetFirstIndex(Char[] colors, char target)
		{
			for (var i = 0; i < colors.Length; i++)
			{
				if (colors[i] == target)
				{
					return i;
				}
			}

			// 来ない
			return colors.Length - 1;
		}

		static int GetLastIndex(Char[] colors, char target)
		{
			for (var i = colors.Length - 1; i >= 0; i--)
			{
				if (colors[i] == target)
				{
					return i;
				}
			}

			// 来ない
			return 0;
		}


		static int GetIndex(Char[] colors, char target)
		{
			if(target == 'W')
			{
				for(var i = 0; i < colors.Length; i++)
				{
					if(colors[i] == target)
					{
						return i;
					}
				}
			}
			else
			{
				for (var i = colors.Length - 1; i >= 0; i--)
				{
					if (colors[i] == target)
					{
						return i;
					}
				}
			}

			// 来ない
			return 0;
		}
	}
}