/*
    Author: Anthony Ngene
    Created: 8/2/20 - 1:28 PM
*/

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

public class Main {
    // If a snake comes into your house, first kill the snake before asking where it came from. - African Proverb
    static char[] arr;
    public static void main(String[] args) throws IOException {
        in = new FastScanner();

        int n = in.intNext();
        arr = in.next().toCharArray();

        out.println(solve());

        out.close();
    }

    static long solve() {
        int n = arr.length;
        int ans = 0;

        int w = getFirstWhite(0);
        if (w >= n) return ans;
        int r = getFirstRed(n - 1);
//        out.pp(w, r);

        while (w < r) {
            ans++;
            u.swap(arr, w, r);
            w = getFirstWhite(w);
            r = getFirstRed(r);

//            out.println(Arrays.toString(arr));
//            out.pp(w, r);
        }

        return ans;
    }

    static int getFirstWhite(int idx) {
        while (idx < arr.length && arr[idx] != 'W') idx++;
        return idx;
    }
    static int getFirstRed(int idx) {
        while (idx >= 0 && arr[idx] != 'R') idx--;
        return idx;
    }

private static final FastWriter out = new FastWriter();
private static FastScanner in;
static ArrayList<Integer>[] adj;
private static long e97 = (long)1e9 + 7;
static class FastWriter {
    private static final int IO_BUFFERS = 128 * 1024;
    private final StringBuilder out;
    public FastWriter() {
        out = new StringBuilder(IO_BUFFERS);
    }
    public FastWriter p(Object object) {
        out.append(object);
        return this;
    }
    public FastWriter p(String format, Object... args) {
        out.append(String.format(format, args));
        return this;
    }
    public FastWriter pp(Object... args) {
        for (Object ob : args) {
            out.append(ob).append(" ");
        }
        out.append("\n");
        return this;
    }
    public FastWriter println(Object object) {
        out.append(object).append("\n");
        return this;
    }
    public void toFile(String fileName) throws IOException {
        BufferedWriter writer = new BufferedWriter(new FileWriter(fileName));
        writer.write(out.toString());
        writer.close();
    }
    public void close() throws IOException {
        System.out.print(out);
    }
}
static class FastScanner {
    private InputStream sin = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    public FastScanner(){}
    public FastScanner(String filename) throws FileNotFoundException {
        File file = new File(filename);
        sin = new FileInputStream(file);
    }
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = sin.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long longNext() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b) || b == ':'){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int intNext() {
        long nl = longNext();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double doubleNext() { return Double.parseDouble(next());}
    public long[] nextLongArray(final int n){
        final long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = longNext();
        return a;
    }
    public int[] nextIntArray(final int n){
        final int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = intNext();
        return a;
    }
    public ArrayList<Integer>[] getAdj(int n) {
        ArrayList<Integer>[] adj = new ArrayList[n + 1];
        for (int i = 1; i <= n; i++) adj[i] = new ArrayList<>();
        return adj;
    }

    public ArrayList<Integer>[] adjacencyList(int n, int m) throws IOException {
        adj = getAdj(n);
        for (int i = 0; i < m; i++) {
            int a = intNext(), b = intNext();
            adj[a].add(b);
            adj[b].add(a);
        }
        return adj;
    }
}
static class u {
    public static int upperBound(long[] array, long obj) {
        int l = 0, r = array.length - 1;
        while (r - l >= 0) {
            int c = (l + r) / 2;
            if (obj < array[c]) {
                r = c - 1;
            } else {
                l = c + 1;
            }
        }
        return l;
    }
    public static int upperBound(ArrayList<Long> array, long obj) {
        int l = 0, r = array.size() - 1;
        while (r - l >= 0) {
            int c = (l + r) / 2;
            if (obj < array.get(c)) {
                r = c - 1;
            } else {
                l = c + 1;
            }
        }
        return l;
    }
    public static int lowerBound(long[] array, long obj) {
        int l = 0, r = array.length - 1;
        while (r - l >= 0) {
            int c = (l + r) / 2;
            if (obj <= array[c]) {
                r = c - 1;
            } else {
                l = c + 1;
            }
        }
        return l;
    }
    public static int lowerBound(ArrayList<Long> array, long obj) {
        int l = 0, r = array.size() - 1;
        while (r - l >= 0) {
            int c = (l + r) / 2;
            if (obj <= array.get(c)) {
                r = c - 1;
            } else {
                l = c + 1;
            }
        }
        return l;
    }
    public static void print(long[] arr) {
        System.out.println(Arrays.toString(arr));
    }
    public static void print(int[] arr) {
        System.out.println(Arrays.toString(arr));
    }
    public static void print(char[] arr) {
        System.out.println(Arrays.toString(arr));
    }
    static <T> T[][] deepCopy(T[][] matrix) {
        return Arrays.stream(matrix).map(el -> el.clone()).toArray($ -> matrix.clone());
    }
    static int[][] deepCopy(int[][] matrix) {
        return Arrays.stream(matrix).map(int[]::clone).toArray($ -> matrix.clone());
    }
    private static void sort(int[][] arr){
        Arrays.sort(arr, Comparator.comparingDouble(o -> o[0]));
    }
    private static void sort(long[][] arr){
        Arrays.sort(arr, Comparator.comparingDouble(o -> o[0]));
    }
    private static void rSort(Long[] arr) {
        Arrays.sort(arr, Collections.reverseOrder());
    }
    private static void rSort(Integer[] arr) {
        Arrays.sort(arr, Collections.reverseOrder());
    }
    private static void sortCustom(int[][] arr) {
        Arrays.sort(arr, new Comparator<int[]>() {
            public int compare(int[] a, int[] b) {
                if (a[0] == b[0]) return Integer.compare(a[1], b[1]);
                return Integer.compare(a[0], b[0]);
            }
        });
    }
    public static char[] swap(char data[], int left, int right) {
        char temp = data[left];
        data[left] = data[right];
        data[right] = temp;
        return data;
    }
    public static int[] reverse(int data[], int left, int right) {
        while (left < right) {
            int temp = data[left];
            data[left++] = data[right];
            data[right--] = temp;
        }
        return data;
    }
    public static int biSearch(int[] dt, int target){
        int left=0, right=dt.length-1;
        int mid=-1;
        while(left<=right){
            mid = (right+left)/2;
            if(dt[mid] == target) return mid;
            if(dt[mid] < target) left=mid+1;
            else right=mid-1;
        }
        return -1;
    }
    public static int biSearchMax(long[] dt, long target){
        int left=-1, right=dt.length;
        int mid=-1;

        while((right-left)>1){
            mid = left + (right-left)/2;
            if(dt[mid] <= target) left=mid;
            else right=mid;
        }
        return left;
    }
    public static int biSearchMaxAL(ArrayList<Integer> dt, long target){
        int left=-1, right=dt.size();
        int mid=-1;

        while((right-left)>1){
            mid = left + (right-left)/2;
            if(dt.get(mid) <= target) left=mid;
            else right=mid;
        }
        return left;
    }
    private static void fill(boolean[] ob, boolean res){    for(int i=0; i<ob.length; i++){ ob[i] = res;    }}
    private static void fill(int[] ob, int res){            for(int i=0; i<ob.length; i++){ ob[i] = res;    }}
    private static void fill(long[] ob, long res){          for(int i=0; i<ob.length; i++){ ob[i] = res;    }}
    private static void fill(char[] ob, char res){          for(int i=0; i<ob.length; i++){ ob[i] = res;    }}
    private static void fill(double[] ob, double res){      for(int i=0; i<ob.length; i++){ ob[i] = res;    }}
    private static void fill(boolean[][] ob,boolean res){for(int i=0;i<ob.length; i++){ for(int j=0; j<ob[0].length; j++){  ob[i][j] = res; }}}
    private static void fill(int[][] ob, int res){      for(int i=0; i<ob.length; i++){ for(int j=0; j<ob[0].length; j++){  ob[i][j] = res; }}}
    private static void fill(long[][] ob, long res){    for(int i=0; i<ob.length; i++){ for(int j=0; j<ob[0].length; j++){  ob[i][j] = res; }}}
    private static void fill(char[][] ob, char res){    for(int i=0; i<ob.length; i++){ for(int j=0; j<ob[0].length; j++){  ob[i][j] = res; }}}
    private static void fill(double[][] ob, double res){for(int i=0; i<ob.length; i++){ for(int j=0; j<ob[0].length; j++){  ob[i][j] = res; }}}
    private static void fill(int[][][] ob,int res){for(int i=0;i<ob.length;i++){for(int j=0;j<ob[0].length;j++){for(int k=0;k<ob[0][0].length;k++){ob[i][j][k]=res;}}}}
    private static void fill(long[][][] ob,long res){for(int i=0;i<ob.length;i++){for(int j=0;j<ob[0].length;j++){for(int k=0;k<ob[0][0].length;k++){ob[i][j][k]=res;}}}}
    private static void fill_parent(int[] ob){ for(int i=0; i<ob.length; i++) ob[i]=i; }
    private static int arrMin(int... ins){
        int min = ins[0];
        for(int i=1; i<ins.length; i++){    if(ins[i] < min) min = ins[i];  }
        return min;
    }
    private static int arrMax(int... ins){
        int max = ins[0];
        for(int i=1; i<ins.length; i++){    if(ins[i] > max) max = ins[i];  }
        return max;
    }
    private static long arrMin(long... ins){
        long min = ins[0];
        for(int i=1; i<ins.length; i++){    if(ins[i] < min) min = ins[i];  }
        return min;
    }
    private static long arrMax(long... ins){
        long max = ins[0];
        for(int i=1; i<ins.length; i++){    if(ins[i] > max) max = ins[i];  }
        return max;
    }
    private static boolean same3(long a, long b, long c){
        if(a!=b) return false;
        if(b!=c) return false;
        if(c!=a) return false;
        return true;
    }
    private static boolean dif3(long a, long b, long c){
        if(a==b) return false;
        if(b==c) return false;
        if(c==a) return false;
        return true;
    }
    private static double hypotenuse(double a, double b){
        return Math.sqrt(a*a+b*b);
    }
    private static long factorial(int n) {
        long ans=1;
        for(long i=n; i>0; i--){ ans*=i; }
        return ans;
    }
    private static long facMod(int n, long mod) {
        long ans=1;
        for(long i=n; i>0; i--) ans = (ans * i) % mod;
        return ans;
    }
    private static long lcm(long m, long n){
        long ans = m/gcd(m,n);
        ans *= n;
        return ans;
    }
    private static long gcd(long m, long n) {
        if(m < n) return gcd(n, m);
        if(n == 0) return m;
        return gcd(n, m % n);
    }
    private static boolean isPrime(long a){
        if(a==1) return false;
        for(int i=2; i<=Math.sqrt(a); i++){  if(a%i == 0) return false;  }
        return true;
    }
    static long modInverse(long a, long mod) {
    /* Fermat's little theorem: a^(MOD-1) => 1
    Therefore (divide both sides by a): a^(MOD-2) => a^(-1) */
        return binpowMod(a, mod - 2, mod);
    }
    static long binpowMod(long a, long b, long mod) {
        long res = 1;
        while (b > 0) {
            if (b % 2 == 1) res = (res * a) % mod;
            a = (a * a) % mod;
            b /= 2;
        }
        return res;
    }
    private static int getDigit2(long num){
        long cf = 1;    int d=0;
        while(num >= cf){   d++;    cf = 1<<d;  }
        return d;
    }
    private static int getDigit10(long num){
        long cf = 1;    int d=0;
        while(num >= cf){   d++;    cf*=10;     }
        return d;
    }
    private static boolean isInArea(int y, int x, int h, int w){
        if(y<0) return false;
        if(x<0) return false;
        if(y>=h) return false;
        if(x>=w) return false;
        return true;
    }
    private static ArrayList<Integer> generatePrimes(int n) {
        int[] lp = new int[n + 1];
        ArrayList<Integer> pr = new ArrayList<>();
        for (int i = 2; i <= n; ++i) {
            if (lp[i] == 0) {
                lp[i] = i;
                pr.add(i);
            }
            for (int j = 0; j < pr.size() && pr.get(j) <= lp[i] && i * pr.get(j) <= n; ++j) {
                lp[i * pr.get(j)] = pr.get(j);
            }
        }
        return pr;
    }
    static long nPrMod(int n, int r, long MOD) {
        long res = 1;
        for (int i = (n - r + 1); i <= n; i++) {
            res = (res * i) % MOD;
        }
        return res;
    }
    static long nCrMod(int n, int r, long MOD) {
        long rFactorial = nPrMod(r, r, MOD);
        long first = nPrMod(n, r, MOD);
        long second = binpowMod(rFactorial, MOD-2, MOD);
        return  (first * second) % MOD;
    }
    static void printBitRepr(int n) {
        StringBuilder res = new StringBuilder();
        for (int i = 0; i < 32; i++) {
            int mask = (1 << i);
            res.append((mask & n) == 0 ? "0" : "1");
        }
        out.println(res);
    }
    static int setKthBitToOne(int n, int k) { return (n | (1 << k)); } // zero indexed
    static int setKthBitToZero(int n, int k) { return (n & ~(1 << k)); }
    static int invertKthBit(int n, int k) { return (n ^ (1 << k)); }
    static boolean isPowerOfTwo(int n) { return (n & (n - 1)) == 0; }
}
}


Measured memory_bytes: 34884