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.*;

public class Main{
	public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
      	int n = Integer.parseInt(sc.next());
      	ArrayList<String> list = new ArrayList<>();
      	char[] c = sc.next().toCharArray();
      	for(int cnt = 0;cnt < c.length;cnt++){
        	list.add(String.valueOf(c[cnt]));
        }
      	if(list.indexOf("W") == -1 || list.indexOf("W") > list.lastIndexOf("R") && list.indexOf("R") != -1){
        	System.out.println(-1);
        }else if(list.indexOf("R") == -1){
        	System.out.println(1);
        }else{
          	int w = 0;
          	int r = 0;
          	for(int i = list.size() - 1; i > list.indexOf("R");i--){
            	if(list.get(i).equals("R")){
                	r++;
                }
            }
          	for(int i = 0;i < list.indexOf("R");i++){
            	if(list.get(i).equals("W")){
                	w++;
                }
            }
          	if(r < w){
            	System.out.println(w);
            }else{
            	System.out.println(r);
            }
        }
    }
}