166
rbbbbbbbbbbbbrbrrbbrbbbrbbbbbbbbbbrbbbbbbrbbbrbbbbbrbbbbbbbrbbbbbbbrbbrbbbbbbbbrbbbbbbbbbbbbbbrrbbbrbbbbbbbbbbbbbbrbrbbbbbbbbbbbrbbbbbbbbbbbbbbrbbbbbbbbbbbbbbbbbbbbbb
 7
rrbrbrb
 1
r
 2
bb
 18
rrrrrrrrrrrrrrrrrb
 8
rbbrbrbr
 1
b
 13
rbbbrbrrbrrbb
 100
brbbbrrrbbrbrbbrbbrbbbbrbbrrbbbrrbbbbrbrbbbbbbbbbbbbbbbbrrrrbbbbrrrbbbbbbbrbrrbrbbbbrrrbbbbrbbrbbbrb
 2
rb
 2
rr
 2
br

70
 1
 0
 1
 8
 1
 0
 3
 34
 0
 1
 0

import java.io.*;
import java.net.Inet4Address;
import java.util.*;
public class ad {
    public static void main(String args[]) throws IOException {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader sc = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        int n = sc.nextInt();
        String str = sc.next();
        int r1 = 0, b1 = 0;
        for (int i = 0; i < n; i++) {
            if (str.charAt(i) != 'r' && i % 2 == 0) {
                r1++;
            } else if (str.charAt(i) != 'b' && i % 2 != 0) {
                b1++;
            }
        }
        int x = Math.max(r1, b1);
        r1 = 0;
        b1 = 0;
        for (int i = 0; i < n; i++) {
            if (str.charAt(i) != 'b' && i % 2 == 0) {
                b1++;
            } else if (str.charAt(i) != 'r' && i % 2 != 0) {
                r1++;
            }
        }
        out.print(Math.min(x, Math.max(r1, b1)));


        out.close();
    }

    public static void shuffle(long[] arr) {
        int n = arr.length;
        Random rnd = new Random();
        for (int i = 0; i < n; ++i) {
            long tmp = arr[i];
            int randomPos = i + rnd.nextInt(n - i);
            arr[i] = arr[randomPos];
            arr[randomPos] = tmp;
        }
    }

    public static void shuffle(int[] arr) {
        int n = arr.length;
        Random rnd = new Random();
        for (int i = 0; i < n; ++i) {
            int tmp = arr[i];
            int randomPos = i + rnd.nextInt(n - i);
            arr[i] = arr[randomPos];
            arr[randomPos] = tmp;
        }
    }

    public static int gcd(int x, int y) {
        if (y == 0)
            return x;
        return gcd(y, x % y);
    }

    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());
        }

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

        public double nextDouble() {
            return Double.valueOf(next());
        }

        String nextLine() throws IOException {
            return reader.readLine();
        }

    }
}
