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;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class Program {
    static void Main (string[] args) {
        int N = int.Parse (Console.ReadLine ());
        char[] str = Console.ReadLine ().ToCharArray ();
        bool[] arr = new bool[N];
        int R = 0;
        int W = 0;
        for (int i = 0; i < N; i++) {
            if (str[i] == 'R') {
                arr[i] = true;
                R++;
            } else {
                arr[i] = false;
                W++;
            }
        }
        int ans = 0;
        if (W < R) {
            int cnt = 0;
            for (int i = 0; i < W; i++) {
                if (arr[i]) {
                    cnt++;
                }
            }
            ans = W - cnt;
        } else {
            int cnt = 0;
            for (int i = 0; i < R; i++) {
                if (arr[i]) {
                    cnt++;
                }
            }
            ans = R - cnt;
        }
        if (ans < 0) {
            ans = 0;
        }
        Console.WriteLine (ans);

    }

}

//dotnet run --project test