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: Runtime Error)

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	private static Scanner sc;
	
	public static void main(String[] args) {
		sc =  new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		String colors = sc.next();
		String[] colorArray = colors.split("");
		List<String> stoneColorList = new ArrayList<>();
		for (int i = 0; i<n; i++) {
			//W or R
			stoneColorList.add(colorArray[i]);
		}
		
		boolean b = true;
		int result = 0;
		while(b) {
			List<String> subListA = stoneColorList.subList(0, n/2);
			System.out.println(subListA);
			List<String> subListB = stoneColorList.subList(n/2, n);
			System.out.println(subListB);
			//左側にWがいくつある
			int subA = culcColor(subListA,"W");
			System.out.println(subA);
			//左側にRがいくつある
			int subB = culcColor(subListB,"R");
			System.out.println(subB);
			if (subB != 0) {
				result += subA;
			}
			if (subA<subB) {
				stoneColorList = subListB;
				n = n/2+1;
			} else {
				b = false;
			}
			
		}
		System.out.println(result);
		
		
	}
	
	private static int culcColor(List<String> list, String col) {
		int result = 0;
		for (String color : list) {
			if (col.equals(color)) {
				result++;
			}
		}
		return result;
	}
}