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: Wrong Answer)

import java.util.*;

class Main{
    void solve(){
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        String s = scan.next();
        char[] c = new char[n];
        for(int i = 0; i < n; i++) c[i] = s.charAt(i);
        int out1 = 0;
        int cw = 0;
        for(int i = 0; i < n; i++){
            if(c[i] == 'W') cw++;
        }
        if(cw == 0 || cw == n){
            System.out.println(0);
            return;
        }
        out1 = Math.min(cw, n - cw);
        int out2 = 0;
        int w = 1;
        for(int i = 1; i < n; i++){
            if(c[i - 1] == 'W' && c[i] == 'W') w++;
            else{
                if(c[i - 1] != 'R' && c[i] == 'R') w = 0;
                int r = 0;
                for(int j = n - 1; j >= i; j--){
                    if(c[j] == 'R'){
                        r++;
                        c[j] ='W';
                        c[i - 1] = 'R';
                    }
                    if(r >= w) break;
                }
                out2 += r;
                w = 1;
            }
        }
        System.out.println(Math.min(out1, out2));
        scan.close();
    }
    public static void main(String[] args){
        new Main().solve();
    }
}