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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.LinkedList;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scn=new Scanner(System.in);
		int n =scn.nextInt();
		String s=scn.next();
		char []arr = s.toCharArray();
	    LinkedList<Integer> al =new LinkedList<>();
	    int cnt=0;
		for(int i=0;i<arr.length;i++){
			if(arr[i]=='R'){
				al.addFirst(i);
				cnt++;
			}
		}
		int ans=0;
		for(int i=0;i<arr.length;i++){
			if(i<cnt){
				if(arr[i]=='W'){
					ans++;
					int idx = al.removeFirst();
					arr[i]='R';
					arr[idx]='W';
				}
			}
		}
		System.out.println(ans);

	}

}