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 static System.Console;
using System.Collections.Generic;
using System.Linq;

namespace AC
{
   public class shiokara
   {
      static void Main(string[] args)
      {
         int N = int.Parse(ReadLine());
         string C = ReadLine();
         int cnt = 0, wpos;
         var windex = new Queue<int>();
         for(int i = 0; i < C.Length; i++){
             if(C[i] == 'W'){
                 windex.Enqueue(i);
             }
         }

         //操作する必要がない時
         if(windex.Count != 0){
             wpos = windex.Peek();
         }else{
             WriteLine("0");
             return;
         }

         for(int i = C.Length - 1; i > wpos; i--){
             if(C[i] == 'R'){
                 if(C[i-1] == 'W'){
                     C.ChangeCharAt(i, 'W');
                     C.ChangeCharAt(windex.Dequeue(), 'R');
                     cnt++;
                 }
             }
             if(windex.Count == 0) break;
         }
         WriteLine(cnt);
      }
   }

   public static class String
   {
       public static string ChangeCharAt(this string str, int index, char newChar)
       {
           return str.Remove(index, 1).Insert(index, newChar.ToString());
        }
    }
}