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.*;
public class Main
{
	public static void main(String[] args) {
	Scanner sc=new Scanner(System.in);
	int x=sc.nextInt();
	
	String str=sc.next();
	ArrayList<Integer> arr=new ArrayList<Integer>();
	int len=str.length();
	
	for(int z=0;z<len;z++)
	{
	  if(str.charAt(z)=='W')
	    arr.add(1);
	   else 
	     arr.add(0);
	} 
	int i=0,j=len-1,count=0;
	while(i<j)
	{
	    while(i<len && j>0 && arr.get(i)==0)
	    {
	        i++;
	    }
	    while(j>=0 && i<len-1 && arr.get(j)==1)
	    {
	        j--;
	    }
	    if(i<j )
	    {
	        arr.set(i,0);
	        arr.set(j,1);
	       
	        count++;
	    }
	}
	System.out.println(count);
}
}