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

namespace AtCoder
{
    class Program
    {
        private static long mod = 1000000007;
        static void Main(string[] args)
        {
            var n = int.Parse(ReadLine());
            //var line = ReadLine().Split(' ').Select(value => long.Parse(value)).ToArray();

            var c = new StringBuilder(ReadLine());

            var left = 0;
            var right = n - 1;
            var count = 0;
            while(left < right)
            {
                if(c[left] == 'W' && c[right] == 'R')
                {
                    c[left] = 'R';
                    c[right] = 'W';
                    left++;
                    right--;
                    count++;
                }
                else
                {
                    if(c[left] == 'R')
                    {
                        left++;
                    }
                    if(c[right] == 'W')
                    {
                        right--;
                    }
                }
            }

            for(var i = 1; i < n; i++)
            {
                if(c[i - 1] == 'W' && c[i] == 'R')
                {
                    c[i - 1] = 'R';
                    count++;
                }
            }

            WriteLine(count);
        }
    }
}