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: Runtime Error)

import java.io.*;
import java.util.*;

public class Main {

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tok;

        public InputReader(InputStream inputStream) {
            reader = new BufferedReader(new InputStreamReader(inputStream));
            tok = null;
        }

        public InputReader(String inputFile) throws FileNotFoundException {
            reader = new BufferedReader(new FileReader(inputFile));
            tok = null;
        }

        public String nextLine() {
            String c = "";
            try {
                c = reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }

            return c;
        }

        public String next() {
            while (tok == null || !tok.hasMoreTokens()) {
                try {
                    tok = new StringTokenizer(nextLine());
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }

            return tok.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }
    }

    public static void main(String args[]) {
        InputStream inputstream = System.in;
        OutputStream outputstream = System.out;
        InputReader in = new InputReader(inputstream);
        PrintWriter out = new PrintWriter(outputstream);
        Task solver = new Task();
        solver.solve(in, out);
        out.flush();
    }

    static class Task {
        public void solve(InputReader in, PrintWriter out) {
            int n = in.nextInt();
            String colors = in.next();
            int whites = 0, swaps = 0;
            for (char c : colors.toCharArray()) {
                if (c == 'W')
                    whites++;
            }
            if (whites == 0)
                out.println(swaps);
            else {
                int i = 0, j = n - 1;
                while (i < j) {
                    if (colors.charAt(i) == 'W' && colors.charAt(j) == 'R') {
                        swaps++;
                        i++;
                        j--;
                    } else if (colors.charAt(i) == 'R')
                        i++;
                    else if (colors.charAt(j) == 'W')
                        j++;
                }
                out.println(swaps);
            }

        }
    }
}