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)

class Main{

	public static void main (String[] args) {

		Main main = new Main();

		java.util.Scanner sc = new java.util.Scanner(System.in);

		int N = sc.nextInt();
		char[] list = new char [N];
		String color = sc.next();

		for (int index=0; index<N; index++) {
			list[index] = color.charAt(index);
		}
		int count = 0;
		boolean flag = true;

		while (flag) {
			int r = main.findR (N, list);
			int w = main.findW(N, list);

			if (r==N||w==N) {
				break;
			} else if (r<w) {
				break;
			}

			list[r]='W';
			list[w]='R';
			count++;
		}
		System.out.println(count);

		sc.close();;

	}

	int findR (int N, char[] list) {
		for (int index=N-1; index>=0; index--) {
			if (list[index]=='R') {
				return index;
			}
		}
		return N;
	}
	int findW (int N, char[] list) {
		for (int index=0; index<N; index++) {
			if (list[index]=='W') {
				return index;
			}
		}
		return N;
	}


}