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.IOException;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));

        String line = stdin.readLine();
        int n = Integer.parseInt(line);
        line = stdin.readLine();
        char[] c = line.toCharArray();

        int start = Integer.MAX_VALUE;
        for (int i = 0; i < n; i++) {
            if (c[i] == 'W' && start == Integer.MAX_VALUE) {
                start = i;
                break;
            }
        }
        int end = -1;
        for (int i = n - 1; i >= 0; i--) {
            if (c[i] == 'R' && end == -1) {
                end = i;
                break;
            }
        }

        int cnt = 0;
        while (start < end) {
            c[start++] = 'R';
            c[end--] = 'W';
            cnt++;

            for (int i = start; i <= end; i++) {
                if (c[i] == 'W') {
                    start = i;
                    break;
                }
            }
            for (int i = end; i >= start + 1; i--) {
                if (c[i] == 'R') {
                    end = i;
                    break;
                }
            }
        }

        int cntw = 0;
        for (int i = 0; i < n; i++) {
            if (c[i] == 'W') {
                cntw++;
            }
        }

        if (cntw < cnt) {
            System.out.println(cntw);
        } else {
            System.out.println(cnt);
        }
    }
}