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.util.*;
import java.io.*;
import java.math.*;
public class loser
{
    static class InputReader {
        public BufferedReader br;
        public StringTokenizer token;
        public InputReader(InputStream stream)
        {
            br=new BufferedReader(new InputStreamReader(stream),32768);
            token=null;
        }

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

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

        public long nextLong()
        {
            return Long.parseLong(next());
        }
    }
    static class card{
        String s;
        int l;
        public card(String s,int i)
        {
            this.s=s;
            this.l=i;
        }
    }
    static class sort implements Comparator<card>
    {
        public int compare(card o1,card o2)
        {
            if(o1.l!=o2.l)
                return (o1.l-o2.l);
            else
                return o1.s.compareTo(o2.s);
        }
    }
    static void shuffle(int a[])
    {
        List<Integer> l=new ArrayList<>();
        for(int i=0;i<a.length;i++)
            l.add(a[i]);
        Collections.shuffle(l);
        for(int i=0;i<a.length;i++)
            a[i]=l.get(i);
    }

    /*static long gcd(long a,long b)
    {
    if(b==0)
    return a;
    else
    return gcd(b,a%b);
    }
    static boolean valid(int i,int j,int r,int c)
    {
    if(i<r && i>=0 && j<c && j>=0)
    return true;
    else
    return false;
    }*/
    static class Pair
    {
        int a;int b;
        public Pair(int a,int b)
        {
            this.a =a;
            this.b =b;
        }
    }
    static boolean beauty(int n)
    {
        String s1=Integer.toBinaryString(n);
        int f=1,o=0,z=0;boolean res=true;
        for(int i=0;i<s1.length();i++)
        {
            if(f==1 && s1.charAt(i)=='1')
                o++;
            else if(s1.charAt(i)=='0' && f==1)
            {
                z++;
                f=0;
            }
            else if(s1.charAt(i)=='0' && f==0)
                z++;
            else if(f==0 && s1.charAt(i)=='1')
            {
                res=false;
                break;
            }
        }
        if((o-1)!=z)
            res=false;
        return res;
    }

    public  static void main(String[] args) 
    {
        InputReader sc=new InputReader(System.in);
        int n=sc.nextInt();
        char c[]=sc.next().toCharArray();
        char c1[]=new char[n];
        char c2[]=new char[n];
        for(int i=0;i<n;i++)
        {
            if(i%2==0)
            {
                c1[i]='r';
                c2[i]='b';
            }
            else
            {
                c1[i]='b';
                c2[i]='r';
            }
        }
        int t=2,ans1=0,ans2=0;
        while(t>0)
        {
            if(t%2==0)
            {
                int x=0,y=0;
                for(int i=0;i<n;i++)
                {
                    if(c1[i]!=c[i])
                    {
                        if(c[i]=='r')
                        x++;
                        else
                        y++;
                    }
                }
                ans1=Math.max(x,y);
            }
            else
            {
                int x=0,y=0;
                for(int i=0;i<n;i++)
                {
                    if(c2[i]!=c[i])
                    {
                        if(c[i]=='r')
                        x++;
                        else
                        y++;
                    }
                }
                ans2=Math.max(x,y);
            }
            t--;
        }
        System.out.println(Math.min(ans1,ans2));
    }
}