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: Time Limit Exceeded)

using System;

namespace abc174
{
    class Program
    {
        static void Main(string[] args)
        {
            int N=int.Parse(Console.ReadLine().Trim());
            string p=Console.ReadLine().Trim();
            bool[] pattern=new bool[N];
            for(int i=0;i<N;i++){
                if(p[i]=='W') pattern[i]=false;
                if(p[i]=='R') pattern[i]=true;
            }
            int res=0;
            int red,white;
            while(search(pattern,out red,out white)){
                int c1=white;
                int c2=red;
                if(c1<c2){
                    bool tmp=pattern[c1];
                    pattern[c1]=pattern[c2];
                    pattern[c2]=tmp;
                }
                res++;
            }

            Console.WriteLine(res);

        }
        static bool search(bool[] p,out int red,out int white){
            bool scan=false;
            bool res=false;
            red=-1;
            white=-1;
            int count=-1;
            foreach(var pp in p){
                count++;
                if(pp==false && scan==false){
                    scan=true;
                    white=count;
                    continue;
                }
                if(scan==true && pp==true){
                    res=true;
                    red=count;
                }

            }

            return res;
        }
    }
}