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: Wrong Answer)

using System;
public class Hello{
    public static void Main()
    {
        long N = long.Parse(System.Console.ReadLine());
        string s = System.Console.ReadLine();
        var c = s.ToCharArray();
        int count = 0; 
        int w = 0;
        int r = 0;
        for (int i = 1; i < N; i++)
        {
            if (c[i] == 'R')
            {
                r++;
            }
            if (c[i - 1] == 'W')
            {
                w++;
                if (c[i] == 'R')
                {
                    count++;
                    count += w;
                }
            }
            else
            {
                w = 0;
            }
        }
        if (count < r)
        {
            Console.WriteLine(count);
        }
        else
        {
            Console.WriteLine(r);
        }
    }
}