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.IO;
using System.Text;
using static Scanner;

class Program
{
    static void Main(string[] args) {
        var writer = new StreamWriter(Console.OpenStandardOutput()) {AutoFlush = false};
        Console.SetOut(writer);
        Solve();
        Console.Out.Flush();
    }
    static void Solve() {
        _ = ScanLine();
        var c = Scan();
        var red = c.Length - c.Replace("W", "").Length;
        var answer = 0;
        for (int i = 0; i < red; i++) {
            if (c[i] == 'W') answer++;
        }
        Console.WriteLine(answer);
    }
}

public static class Scanner
{
    static readonly Stream stream = Console.OpenStandardInput();
    static readonly byte[] buffer = new byte[1024];
    static int length, p;
    static bool isEof;
    private static byte ConsoleIn() {
        if (isEof) return 0;
        if (p < length) return buffer[p++];
        p = 0;
        if ((length = stream.Read(buffer, 0, 1024)) > 0) return buffer[p++];
        isEof = true;
        return 0;
    }
    public static char GetChar() {
        var by = ConsoleIn();
        while ((by < '!' || '~' < by) && !isEof) by = ConsoleIn();
        return (char) by;
    }
    public static string Scan() {
        var sb = new StringBuilder();
        for (var ch = GetChar(); '!' <= ch && ch <= '~'; ch = (char) ConsoleIn()) {
            sb.Append(ch);
        }
        return sb.ToString();
    }
    public static string ScanLine() {
        var sb = new StringBuilder();
        for (var ch = GetChar(); ch != '\n' && ch != 0; ch = (char) ConsoleIn()) {
            if (ch != '\r') sb.Append(ch);
        }
        return sb.ToString();
    }
}