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.Scanner;

public class Main {
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)) {
		
			int n = sc.nextInt();
			
			char[] stones = sc.next().toCharArray();
			
			
			//基本は入れ替えすればよい
			int lp = 0;
			int rp = n-1;
			
			int count = 0;
			while(true) {
				while( stones[lp] == 'R' && lp < n-1) {
					lp++;
				}
				while( stones[rp] == 'W' && rp > 0) {
					rp--;
				}
				
//				System.out.println(lp + " " + rp);
				
				if ( lp >= rp) {
					break;
				}
				
				if ( stones[lp] == 'W' && stones[rp] == 'R' ) { 
					stones[lp] = 'R';
					stones[rp] = 'W';
					count++;
				}

			}
			
			System.out.println(count);
			
		}
	}

}