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: Accepted)

import java.io.*;
import java.util.*;
import java.util.Arrays;
import java.util.Collections;
// import java.io.BufferedReader;
// import java.io.IOException;
// import java.io.InputStreamReader;
// import java.util.ArrayList;
// import java.util.Arrays;
// import java.util.Collections;
// import java.util.List;
// import java.util.HashSet;
// import java.util.Set;
// import java.util.Map;
// import java.util.HashMap;

import java.math.*;

public class Main {
    private static Scanner sc;
    private static final long mod = 1000000007;

    public static void main(final String[] args) {
        final Main instance = new Main();
        sc = instance.new Scanner();
        instance.solve();
    }

    private static String getUpperAlpha(int i) {
        return String.valueOf((char) (i + 64));
    }

    /* i=1 return a */
    private static String getLowerAlpha(int i) {
        return String.valueOf((char) (i + 96));
    }

    private class Scanner {
        String[] s;
        int i;
        BufferedReader br;
        String regex = " ";

        public Scanner() {
            s = new String[0];
            i = 0;
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        @Override
        protected void finalize() throws Throwable {
            try {
                super.finalize();
            } finally {
                destruction();
            }
        }

        private void destruction() throws IOException {
            if (br != null)
                br.close();
        }

        public String next() throws IOException {
            if (i < s.length)
                return s[i++];
            String st = br.readLine();
            while (st == "")
                st = br.readLine();
            s = st.split(regex, 0);
            i = 0;
            return s[i++];
        }

        public int nextInt() throws NumberFormatException, IOException {
            return Integer.parseInt(next());
        }

        public Long nextLong() throws NumberFormatException, IOException {
            return Long.parseLong(next());
        }

        public Double nextDouble() throws NumberFormatException, IOException {
            return Double.parseDouble(next());
        }

        public long[] nextLongArray(int n) throws NumberFormatException, IOException {
            long[] ret = new long[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextLong();
            }
            return ret;
        }

        public int[] nextIntArray(int n) throws NumberFormatException, IOException {
            int[] ret = new int[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextInt();
            }
            return ret;
        }
    }

    private void solve() {
        try {
            int n = sc.nextInt();
            String c =sc.next();
            
            int index = 0;
            int wCount = 0;
            int rCount = 0;
            while(true) {
                index = c.indexOf("W", index) + 1;
                if (index == 0) break;
                wCount++;
            }
            rCount=c.length()- wCount;

            int twCount = 0;
            int trCount = 0;
            
            for (int i = 0; i < n; i++) {
                if(twCount>= (rCount-trCount)) break;
                if(c.substring(i,i+1).equals("W")){
                    twCount++;
                }else{
                    trCount++;
                }
            }
            System.out.println(rCount-trCount);

        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}