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: Accepted)

using System;

class Program
{
    static void Main(string[] args)
    {
        int i, j;

        int n = int.Parse(Console.ReadLine());
        string strInput = Console.ReadLine();
        char[] cInput = strInput.ToCharArray();
        int result = 0;

        int startRNum = 0;  //Rで始まる。
        int endWNum = 0;

        i = 0;
        while (cInput[i] == 'R')
        {
            startRNum++;
            i++;
            if (i >= n) break;
        }

        i = n - 1;
        while(cInput[i] == 'W')
        {
            endWNum++;
            i--;
            if (i < 0) break;
        }

        //R...RW...Wになっている。またはどっちかの色のみ
        if ( startRNum + endWNum == n)
        {
            Console.WriteLine(0);
            return;
        }

        int lPtr = 0;
        int rPtr = n - 1;
        while (true)
        {
            while (cInput[lPtr] == 'R' && lPtr < n) lPtr++;
            while (cInput[rPtr] == 'W' && rPtr >= 0) rPtr--;

            if(lPtr >= rPtr)
            {
                break;
            }

            cInput[lPtr] = 'R';
            cInput[rPtr] = 'W';
            result++;

        }

        Console.WriteLine(result);

        return;
    }
}