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();
    int [] a=new int[n];
    String s=sc.next();
    for(int i=0;i<n;i++)
    {
      char c=s.charAt(i);
      if(c=='W')
      {
        a[i]=1;
      }
      else
      {
        a[i]=0;
      }
    }
    int x=0,y=n-1,c=0;
    while(x<y)
    {
      while(x<n && a[x]==0)
      {
          x++;
      }
      while(y<n && a[y]==1)
      {
        y--;
      }
      if(x<n && y<n && x<y)
      {
      int temp=a[x];
      a[x]=a[y];
      a[y]=temp;
       
      x++;
      y--;
         c++;
      }
     
    }
    System.out.println(c);
  }
}