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;

namespace abc174_d
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = CIn.ReadInt();
            var c = CIn.Read();
            var red = 0;
            var ans = 0;
            for (var i = 0; i < N; i++)
            {
                if (c[i] == 'R')
                {
                    red += 1;
                }
            }
            for(var j = red; j < N; j++)
            {
                if(c[j] == 'R')
                {
                    ans += 1;
                }
            }
            Console.WriteLine(ans);
        }   
    }
    class CIn
    {
        private static readonly Queue<string> fieldBuffer = new Queue<string>();

        public static string Read()
        {
            if (fieldBuffer.Count == 0)
            {
                foreach (var field in Console.ReadLine().Split())
                {
                    fieldBuffer.Enqueue(field);
                }
            }

            return fieldBuffer.Dequeue();
        }

        public static int ReadInt() => int.Parse(Read());
        public static long ReadLong() => long.Parse(Read());
        public static double ReadDouble() => double.Parse(Read());

        public static string[] ReadStringArray(long n)
        {
            var array = new string[n];

            for (var i = 0; i < n; i++)
            {
                array[i] = Read();
            }

            return array;
        }

        public static int[] ReadIntArray(long n)
        {
            var array = new int[n];

            for (var i = 0; i < n; i++)
            {
                array[i] = ReadInt();
            }

            return array;
        }

        public static long[] ReadLongArray(long n)
        {
            var array = new long[n];

            for (var i = 0; i < n; i++)
            {
                array[i] = ReadLong();
            }

            return array;
        }
        public static double[] ReadDoubleArray(long n)
        {
            var array = new double[n];

            for (var i = 0; i < n; i++)
            {
                array[i] = ReadDouble();
            }

            return array;
        }
    }

}