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: Wrong Answer)



import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Scanner;

public final class Main {

    public static void main(String[] args) {
        final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
        final int n = Integer.parseInt(in.nextLine());
        final String word = in.nextLine();
        int red = 0;
        for (char c : word.toCharArray()) {
            if (c == 'R') {
                red++;
            }
        }
        int res = 0;
        for (int i = 0; i < red; i++) {
            if (word.charAt(i) == 'W') {
                res++;
            }
        }
        System.out.println(getSwaps(n, word, red, 'W'));
    }

    private static int getSwaps(int n, String word, int color, char c) {
        int swaps = 0;
        for (int i = 0; i < n; i++) {
            if (word.charAt(i) == c) {
                swaps++;
            }
            color--;
            if (color == 0) {
                break;
            }
        }
        return swaps;
    }
}