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: Wrong Answer)

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// AtCoder_D
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();
		int ans = 0;

		sc.nextLine();
		String C = sc.nextLine();

		int cntR = 0;
		int cntW = 0;
		int mkW = 0;

		//RRWWにする

		for(int i=0; i<N; i++) {
			if(C.charAt(i) == 'R') {
				cntR++;
			} else {
				cntW++;
				mkW = i;
			}
		}
		if(cntR != 0 && cntW != 0) {
			// 一番右のWから先を全部Wにする（入れ替え）
			ans = N - 1 - mkW;
	
			// 一番右のRの位置を求める
			int mkR = mkW;
			for(int i=mkW; i>=0; i--) {
				if(C.charAt(i) == 'R') {
					mkR = i;
					break;
				}
			}
	
			// 残りのWをRにする
			ans += (mkR + 1 - cntR);
		}

		// 出力
		System.out.println(ans);
	    // Scannerクラスのインスタンスをクローズ
		sc.close();
	}
}