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: Time Limit Exceeded)

import java.util.Scanner;

public class Main{
	
	public static void main(String[] args){
		
		Scanner s = new Scanner(System.in);
		
		int numberOfStones = s.nextInt();
		
		String stones = s.next();
		
		int counter = 0;
		
		
		
		
			
			for(int i = 0; i < stones.length() - 1; i++){
				
				int positionOfR = positionOfLastR(stones);
				
				if(stones.charAt(i) == 'W' && positionOfR > i){
					
					stones = swap(stones, i, positionOfR);
					counter++;
					
				}
				
			}
			
		
		
		System.out.println(counter);
		
		}
		
		public static boolean isOkay(String str){
			
			for(int i = 0; i < str.length() - 1; i++){
				if(str.charAt(i) == 'W' && str.charAt(i+1) == 'R'){
					return false;
				}
			}
			
			return true;
		}
	
		public static int positionOfLastR(String str){
			
			for(int i = str.length() - 1; i >= 0; i--){
				if(str.charAt(i) == 'R'){
					return i;
				}
			
				if(i == 0){
					return -1;
				}
			}
			
			return 0;
		}
	
		public static String swap(String s, int l, int m){
		
		char[] charArray = s.toCharArray();
		char temp = charArray[l];
		charArray[l] = charArray[m];
		charArray[m] = temp;
		String returnString = "";
		
		for(int i = 0; i < charArray.length; i++){
			returnString += charArray[i];
		}
			
		return returnString;	
		}
		
	
}