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.*;
public class Main{
	public static void main(String[] args)
	{
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		String s=sc.next();
		char a[]=s.toCharArray();
		int i=0;
		int j=n-1;
		int count=0;
		int flag=0;
		int flag2=0;
		int flag3=0;
		while(i<=j)
		{
			
			while(a[i]=='R'&&i<j)
			{
				flag3=1;
				i++;
			}
			if(i==n-1)
			{
				break;
			}
			while(a[j]=='W')
			{
				flag2=1;
				j--;
			}
			if(j==0)
			{
				break;
			}
			
			char c=a[i];
			a[i]=a[j];
			a[j]=c;
			flag2=0;
			flag3=0;
		
			count++;
			
			
			i++;
			j--;
			if(i==j)
			{
				break;
			}
		}
		System.out.println(count);
	}

}