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.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
class Program
{
    static void Main(string[] args)
    {
            // 入力
            string sS = System.Console.ReadLine().Trim();
            int iN = int.Parse(sS);                                                     // 総数
            sS = System.Console.ReadLine().Trim();

            System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
            sw.Start();

            // 解析
            List<int> lstOut = new List<int>();
            int iWAt = iN - sS.Replace("W", "").Length;                                 // 白の数
            int iRAt = iN - iWAt;                                                       // 赤の数
            int iWLc = 0;                                                               // 仕切より左の白の数
            int iRRc = iRAt;                                                            // 仕切より右の赤の数
            lstOut.Add(Math.Max(0, iRAt));                                              // 仕切最左端の位置の仕切の右側のWの数と左側のRの数の大きい方
            for (int i = 0; i < iN; i++)                                                // 仕切の位置を石の間に置く
            {
                string sOne = sS.Substring(i, 1);                                       // 前回の仕切の位置から右へ移動する石
                switch (sOne)
                {
                    case "W": iWLc += 1; break;                                         // WだったらiWLc+1
                    case "R": iRRc -= 1; break;                                         // RだったらiRRc+1
                }
                lstOut.Add(Math.Max(iWLc, iRRc));                                       // 石の間の仕切位置の右側のWの数と左側のRの数の大きい方
            }
            lstOut.Add(Math.Max(iWAt, 0));                                              // 仕切最右端の位置の仕切の右側のWの数と左側のRの数の大きい方
            lstOut.Sort();

            // 出力
            System.Console.WriteLine("{0}", lstOut[0].ToString().Trim());

            sw.Stop();
#if DEBUG
            Console.WriteLine();
            Console.WriteLine("{0}", sw.Elapsed.ToString());
            Console.WriteLine("続行するには何かキーを押してください．．．");
            Console.ReadKey();
#endif
    }

}