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: Time Limit Exceeded)

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        String stones = sc.next();

        int current = n - 1;
        long steps = 0;
        while (stones.indexOf("W") < stones.lastIndexOf("R")
                && stones.contains("W")
                && stones.contains("R")
        ) {
            if (stones.charAt(current) == 'R') {
                if (stones.contains("W")) {
                    stones = replace(stones.indexOf("W"), current, stones);
                } else {
                    stones = changeToW(current, stones);
                }
                steps++;
            }
            current--;
        }
        System.out.println(steps);
    }

    private static String replace(int wIndex, int rIndex, String stones) {
        StringBuilder builder = new StringBuilder(stones);
        builder.setCharAt(wIndex, 'R');
        builder.setCharAt(rIndex, 'W');
        return builder.toString();
    }

    private static String changeToW(int index, String stones) {
        StringBuilder builder = new StringBuilder(stones);
        builder.setCharAt(index, 'W');
        return builder.toString();
    }
}