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 1.8.0) (judge verdict: Accepted)

import java.util.Scanner;
public class Main {
    public static void main(String[] args){
         Scanner sc = new Scanner(System.in);
         int N = sc.nextInt();
         int wnum = 0;
         int rnum = 0;
         int ans = 0;
         char[] C = new char[N];
         String text = sc.next();
         for(int i = 0 ; i < N ; i++){
                C[i] = text.charAt(i);
                if(C[i] == 'W'){
                    wnum++;
                }else{
                    rnum++;
                }
         }
         if(wnum == 0||rnum == 0){
         }else{ 
            for(int i = rnum ; i < N ; i++){
                if(C[i] == 'R'){
                    ans++;
                }
            }
         }
         System.out.println(ans);
    }
}