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

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
//package anatolyandcockroaches;

import java.io.IOException;
import java.io.InputStream;

/**
 *
 * @author PrudhviRaj
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        // TODO code application logic here
        InputReader ir = new InputReader();
        int n = ir.readInt();
        String s = ir.read();
        int ans = 0;
        int black=0;
        for(int i=0;i<n;i++){
            if(s.charAt(i)=='b'){
                black++;
            }
        }
        int red = n-black;
        
        //first rbrbrbrb............
        int wr=0,wb=0;
        for(int i=0;i<s.length();i++){
            if(i%2==0){
                if(s.charAt(i)=='b'){
                    wb++;
                }
            }
            else{
                if(s.charAt(i)=='r'){
                    wr++;
                }
            }
        }
        int ans1 = Math.max(wr,wb);
        wr=0;
        wb=0;
        for(int i=0;i<s.length();i++){
            if(i%2==0){
                if(s.charAt(i)=='r'){
                    wb++;
                }
            }
            else{
                if(s.charAt(i)=='b'){
                    wr++;
                }
            }
        }
        int ans2 = Math.max(wr,wb);
        System.out.println(Math.min(ans1, ans2));
        
    }
    final static class InputReader {

        byte[] buffer = new byte[8192];
        int offset = 0;
        int bufferSize = 0;
        final InputStream in = System.in;

        public int readInt() throws IOException {
            int number = 0;
            int s = 1;
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }
            if (bufferSize == -1) {
                throw new IOException("No new bytes");
            }
            for (; buffer[offset] < 0x30 || buffer[offset] == '-'; ++offset) {
                if (buffer[offset] == '-') {
                    s = -1;
                }
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (; offset < bufferSize && buffer[offset] > 0x2f; ++offset) {
                number = (number << 3) + (number << 1) + buffer[offset] - 0x30;
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            ++offset;
            return number * s;
        }

        public int[] readIntArray(int n) throws IOException {
            int[] ar = new int[n];
            for (int i = 0; i < n; i++) {
                ar[i] = readInt();
            }

            return ar;
        }

        public int[] readIntArray1(int n) throws IOException {
            int[] ar = new int[n + 1];
            for (int i = 1; i <= n; i++) {
                ar[i] = readInt();
            }

            return ar;
        }

        public long readLong() throws IOException {
            long res = 0;
            int s = 1;
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }
            for (; buffer[offset] < 0x30 || buffer[offset] == '-'; ++offset) {
                if (buffer[offset] == '-') {
                    s = -1;
                }
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (; offset < bufferSize && buffer[offset] > 0x2f; ++offset) {
                res = (res << 3) + (res << 1) + buffer[offset] - 0x30;
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            ++offset;
            if (s == -1) {
                res = -res;
            }
            return res;
        }

        public long[] readLongArray(int n) throws IOException {
            long[] ar = new long[n];

            for (int i = 0; i < n; i++) {
                ar[i] = readLong();
            }

            return ar;
        }

        public String read() throws IOException {
            StringBuilder sb = new StringBuilder();
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }

            if (bufferSize == -1 || bufferSize == 0) {
                throw new IOException("No new bytes");
            }

            for (;
                    buffer[offset] == ' ' || buffer[offset] == '\t' || buffer[offset]
                    == '\n' || buffer[offset] == '\r'; ++offset) {
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (; offset < bufferSize; ++offset) {
                if (buffer[offset] == ' ' || buffer[offset] == '\t'
                        || buffer[offset] == '\n' || buffer[offset] == '\r') {
                    break;
                }
                if (Character.isValidCodePoint(buffer[offset])) {
                    sb.appendCodePoint(buffer[offset]);
                }
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            return sb.toString();
        }

        public String read(int n) throws IOException {
            StringBuilder sb = new StringBuilder(n);
            if (offset == bufferSize) {
                offset = 0;
                bufferSize = in.read(buffer);
            }

            if (bufferSize == -1 || bufferSize == 0) {
                throw new IOException("No new bytes");
            }

            for (;
                    buffer[offset] == ' ' || buffer[offset] == '\t' || buffer[offset]
                    == '\n' || buffer[offset] == '\r'; ++offset) {
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            for (int i = 0; offset < bufferSize && i < n; ++offset) {
                if (buffer[offset] == ' ' || buffer[offset] == '\t'
                        || buffer[offset] == '\n' || buffer[offset] == '\r') {
                    break;
                }
                if (Character.isValidCodePoint(buffer[offset])) {
                    sb.appendCodePoint(buffer[offset]);
                }
                if (offset == bufferSize - 1) {
                    offset = -1;
                    bufferSize = in.read(buffer);
                }
            }
            return sb.toString();
        }
    }
}
