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;
using System.Text;
using System.Collections.Generic;
using System.Collections;
using static System.Math;
using System.Linq;
using static System.Array;

namespace ConsoleApp2
{
    class Program
    {
        static string C;
        static int LW;
        static int RR;
        static int N;

        static void Main(string[] args)
        {
            N = int.Parse(Console.ReadLine());
            //var NA = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();

            C = Console.ReadLine();
            LW = -1;
            RR = N;
            var count = 0;

            while (LW < RR)
            {
                NextLW();
                NextRR();
                //if(fif && LW >= RR)
                //{
                //    break;
                //}
                //else
                //{
                //    fif = false;
                //}
                if (LW >= RR)
                {
                    break;
                }
                count++;
            }

            Console.WriteLine(count);
        }

        static void NextLW()
        {
            LW++;
            while (LW < N && C[LW] == 'R')
            {
                LW++;
            }
        }

        static void NextRR()
        {
            RR--;
            while (RR >= 0 && C[RR] == 'W')
            {
                RR--;
            }
        }
    }


}