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;
public class Hello{
    public static void Main(){
        var N = int.Parse(Console.ReadLine());
        var C = Console.ReadLine();
        var ans = 0;
        for(var i = 0;i<=N;i++){
            if(i >= N) break;
            //Console.WriteLine("i = {0}",i);
            if(C[i] == 'W'){
                for(var n = N;n>i;N--,n--){
                    if(i >= N) break;
                    //Console.WriteLine("N = {0}",n);
                    if(C[n-1] == 'R'){
                        ans++;
                        N--;
                        break;
                    }
                    
                }
            }
        }
        Console.Write(ans);
    }
}