import java.io.InputStream;
import java.io.OutputStream;
import java.util.*;
import java.io.PrintWriter;
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

public class Main{
    //WRWWRWRR     wwwrrrrwwwwrrr
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n = in.nextInt();
        String s = in.toString();
        out.println(s);
        LinkedList<Integer> q = new LinkedList<>();
        for(int i=0;i<n;i++){
            if(s.charAt(i)=='W'){
                q.add(i);
            }
        }
        int res = 0;
        for(int i=n-1;i>=0;i--){
            if(q.size()==0) break;
            if(s.charAt(i)=='R'){
                int v = q.poll();
                if(v>i) break;
                res++;
            }
        }
        out.println(res);


    }
    public static void main(String[] args){
        Scanner scanner = new Scanner(new InputStreamReader(System.in));
        int n  = Integer.parseInt(scanner.nextLine());
        String s = scanner.nextLine();
        LinkedList<Integer> q = new LinkedList<>();
        for(int i=0;i<n;i++){
            if(s.charAt(i)=='W'){
                q.add(i);
            }
        }
        int res = 0;
        for(int i=n-1;i>=0;i--){
            if(q.size()==0) break;
            if(s.charAt(i)=='R'){
                int v = q.poll();
                if(v>i) break;
                res++;
            }
        }
        System.out.println(res);

    }
//    public static void main(String[] args){
//        InputStream inputStream = System.in;
//        OutputStream outputStream = System.out;
//        BufferedReader br = new BufferedReader();
//        InputReader in = new InputReader(inputStream);
//        PrintWriter out = new PrintWriter(outputStream);
//        TaskA solver = new TaskA();
//        int num = in.nextInt();
//        while(num-->0){
//            solver.solve(1, in, out);
//        }
//        solver.solve(1, in, out);
//        out.close();
//    }
    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

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

    }
}


Measured memory_bytes: 49060