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;
public class Hello
{
    public static void Main()
    {
        long N = long.Parse(System.Console.ReadLine());
        string s = System.Console.ReadLine();
        var c = s.ToCharArray();
        long count = N; 
        long w = 0;
        long r = 0;
        char[] result = new char[N];
        for (long i = 0; i < N; i++)
        {
            result[i] = 'W';
        }
        int pos = 0;
        while (pos <= N)
        {
            if (pos > 0)
            {
                result[pos - 1] = 'R';
            }
            for (long i = 0; i < N; i++)
            {

                if (result[i] != c[i])
                {
                    if (c[i] == 'R')
                    {
                        r++;
                    }
                    else
                    {
                        w++;
                    }
                }
                
            }
            count = Math.Min(count, Math.Max(r, w));
            w = 0;
            r = 0;
            pos++;
        }

        Console.WriteLine(count);
    }
}