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.Linq;
using System.Collections.Generic;

namespace ABC
{
    class ProgramD
    {
        static void Main(string[] args)
        {
            var n = long.Parse(Console.ReadLine());
            var s = Console.ReadLine().Select(x => x).ToArray();

            var result = 0;
            var j = s.Length - 1;
            for (var i = 0; i < n; i++)
            {
                if (s[i] == 'W')
                {
                    for (; j > i; j--)
                    {
                        if (s[j] == 'R')
                        {
                            s[i] = 'R';
                            s[j] = 'W';
                            result++;
                            // Console.WriteLine(string.Join(",", i, j));
                            // Console.WriteLine(string.Join("", s));
                            break;
                        }
                    }
                }
            }
            Console.WriteLine(result);
        }
    }
}