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.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace ABC
{
    class ABC174D
    {
        const string FILE = "/tmp/data.txt";

        static void Main(string[] args)
        {
#if DEBUG
            Console.SetIn(new StreamReader(FILE));
#endif
            var N = RI();
            char[] ca = Console.ReadLine().ToArray();

            int nProc = 0;

            int wi = -1;
            int ri = N;
            while (true)
            {
                wi++;
                if (wi == ri)
                {
                    goto LOOP;
                }
                while (ca[wi] == 'R')
                {
                    wi++;
                    if (wi == ri)
                    {
                        goto LOOP;
                    }
                }

                ri--;
                if (wi == ri)
                {
                    goto LOOP;
                }
                while (ca[ri] == 'W')
                {
                    ri--;
                    if (wi == ri)
                    {
                        goto LOOP;
                    }
                }


                ++nProc;
            }
            LOOP:
            Console.WriteLine(nProc);

#if DEBUG
            Console.WriteLine("*** end ***");
            Console.SetIn(Console.In);
            Console.ReadKey();
            Console.WriteLine();
#endif
        }

        static int RI()
        {
            return int.Parse(Console.ReadLine());
        }

        static int[] RIa(int n)
        {
            string[] sa = Console.ReadLine().Split(' ');
            int[] la = new int[n];
            for (int i = 0; i < n; ++i)
            {
                la[i] = int.Parse(sa[i]);
            }
            return la;
        }
    }
}