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 javafx.util.Pair;

import java.io.*;
import java.sql.Array;
import java.util.*;
public class test {
    int max = 0;
    long max1 = 0;
    boolean visited[];
    PrintStream out = System.out;
    int sign[] = {1, 0, -1, 0, 0, 1, 0, -1};
    ArrayList<Integer> queue = new ArrayList<>();

    public static void main(String args[]) throws IOException {
        test obj = new test();
        obj.start();
    }

    void start() throws IOException {
       // Reader sc = new Reader();
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        Scanner sc=new Scanner(System.in);
       int n=sc.nextInt();
        String s=sc.next();
        StringBuilder str=new StringBuilder(s);
        ArrayList<Integer> p=new ArrayList<Integer>();
        ArrayList<Integer> q=new ArrayList<Integer>();
        char c=str.charAt(0);
        int count=0;
        char h='r';
        if(c=='r')
            h='b';
        for(int i=1;i<n;i++)
        {
              if(i%2==0)
              if(str.charAt(i)!=c)
                  p.add(i);
              if(i%2!=0)
                  if(str.charAt(i)!=h)
                      q.add(i);
              if(p.size()!=0&&q.size()!=0)
              {
                //  out.println("asdf");
                  char c1=str.charAt(p.get(0));
                  char c2=str.charAt(q.get(0));
                  str.setCharAt(p.get(0),c2);
                  str.setCharAt(q.get(0),c1);
                  p.remove(0);
                  q.remove(0);
                  count+=1;
              }
        }
      //  out.println(str);
       // out.println(count);
        for(int i=1;i<n;i++)
        {
            if(i%2==0)
                if(str.charAt(i)!=c)
                    count+=1;
            if(i%2!=0)
                if(str.charAt(i)!=h)
                    count+=1;

        }
        out.println(count);
    }


    //<---------------------------------------------------------------------------------------------------------------->//


    //printing 2-d array
    private void println(int[][] a,int n,int m) {
        for(int i=0;i<n;i++)
        {
            for(int j=0;j<m;j++)
                System.out.print(a[i][j]);
            System.out.println();
        }

    }


    //modified dfs for chessboard, matrix
    void dfs(char a[][],int i,int j,char t,int n,int m)
    {
        //visited[i]=true;
        a[i][j]=t;
        if(t=='B')
            t='W';
        else
            t='B';
        for (int c= 0; c < 4; c++) {

            int temp=i+sign[2*c];
            int temp2=j+sign[2*c+1];
            if(temp>=0&&temp<n&&temp2>=0&&temp2<m) {
                if(a[temp][temp2]=='.'&&a[temp][temp2]!='-')
                    dfs(a,temp,temp2,t,n,m);
            }
        }
    }


     //constructing segment tree
    private void constructseg(long[] seg, long[] input, int p, int q, int pos) {
        if(p==q)
        {
            seg[pos]=input[p];
            return;
        }
        int mid=(q+p)/2;
        constructseg(seg,input,p,mid,2*pos+1);
        constructseg(seg,input,mid+1,q,2*pos+2);
        seg[pos]=seg[2*pos+1]+seg[2*pos+2];
    }


    //traversing through a arrayList graph
    void dfs(ArrayList<Integer> node[],int i,int count,int cat[],int k)
    {
        visited[i]=true;
        if(cat[i]==1) {
            count++;
        }
        else
            count=0;
        if(count>k) {
            return;
        }
        if(node[i].size()==1&&i!=0) {
            max += 1;
        }
        for (int j = 0; j < node[i].size(); j++) {
            if(!visited[node[i].get(j)])
                dfs(node, node[i].get(j), count, cat, k);
        }

    }


    //printing arraylist array
    void println(ArrayList<Integer> a[])
    {
          for (ArrayList<Integer> b:a
               ) {
              System.out.println(b.toString());
          }
    }

    //sorting 2 array based on a, Complexity is O(n^2)
    void sort(int a[],int b[],int n)
    {
        for(int i=1;i<n;i++)
        {
            int index=i;
            for(int j=i-1;j>=0;j--)
            {
              //  println(a);
                if(a[index]<a[j])
                {
                    int temp=a[index];
                    int temp2=b[index];
                    a[index]=a[j];
                    b[index]=b[j];
                    a[j]=temp;
                    b[j]=temp2;
                    index=j;

                }
                else
                    break;
            }
        }

    }
     // to create and initialize Integer array
    private int[] intArray(int a, Scanner sc) {
        int temp[]=new int[a];
        for(int i=0;i<a;i++) {
            temp[i]=sc.nextInt();
        }
        return temp;
    }

    private int[] intArray(int a, Reader sc) throws IOException {
        int temp[]=new int[a];
        for(int i=0;i<a;i++) {
            temp[i]=sc.nextInt();
        }
        return temp;
    }// / to create and initialize Long Array
    private long[] longArray(int a, Scanner sc) {
        long temp[]=new long[a];
        for(int i=0;i<a;i++) {
            temp[i]=sc.nextLong();
        }
        return temp;
    }

    // displaying Map contents key--value
    public static void printMap(Map mp) {
        Iterator it = mp.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry pair = (Map.Entry)it.next();
            System.out.println(pair.getKey() + " = " + pair.getValue());
            it.remove(); // avoids a ConcurrentModificationException
        }
    }


    // Return arrayList of minimum factor of number
    // 24= 2*2*2*3;
    private ArrayList<Integer> minFactor(int a) {
        ArrayList<Integer> obj=new ArrayList<>();
        for(int i=2;i<=a;i++)
        {
            while(a!=0&&a%i==0)
            {
                obj.add(i);
                a/=i;
            }
        }
        return obj;
    }


    // check whether the number is prime or not
    public static boolean isPrimeNumber(int i) {
        int factors = 0;
        int j = 1;

        while(j <= i)
        {
            if(i % j == 0)
            {
                factors++;
            }
            j++;
        }
        return (factors == 2);
    }

    // creating and initializing arrayList
    private void initArrayList(ArrayList<Integer>[] tempa) {
        for(int i=0;i<tempa.length;i++)
            tempa[i]=new ArrayList<>();
    }

    // return factorial contents as ArrayList
    private ArrayList<Integer> factdigit(int i)
    {
        ArrayList<Integer> temp=new ArrayList<>();
        for(int j=i;j>1;j--)
        {
            temp.add(j);
        }
        return temp;
    }

    //printing Integer array
    private  void println(int a[])
    {
        for(int i=0;i<a.length;i++)
            System.out.print(a[i]+" ");
    }
    int factorial(int n){
        if (n == 0)
            return 1;
        else
            return((n %1000000007)* (factorial(n-1)%1000000007));
    }
}





class Reader {
    final private int BUFFER_SIZE = 1 << 16;
    private DataInputStream din;
    private byte[] buffer;
    private int bufferPointer, bytesRead;

    public Reader() {
        din = new DataInputStream(System.in);
        buffer = new byte[BUFFER_SIZE];
        bufferPointer = bytesRead = 0;
    }

    public Reader(String file_name) throws IOException {
        din = new DataInputStream(new FileInputStream(file_name));
        buffer = new byte[BUFFER_SIZE];
        bufferPointer = bytesRead = 0;
    }

    public String readLine() throws IOException {
        byte[] buf = new byte[300]; // line length
        int cnt = 0, c;
        while ((c = read()) != -1) {
            if (c == '\n')
                break;
            buf[cnt++] = (byte) c;
        }
        return new String(buf, 0, cnt);
    }

    public int nextInt() throws IOException {
        int ret = 0;
        byte c = read();
        while (c <= ' ')
            c = read();
        boolean neg = (c == '-');
        if (neg)
            c = read();
        do {
            ret = ret * 10 + c - '0';
        } while ((c = read()) >= '0' && c <= '9');

        if (neg)
            return -ret;
        return ret;
    }

    public long nextLong() throws IOException {
        long ret = 0;
        byte c = read();
        while (c <= ' ')
            c = read();
        boolean neg = (c == '-');
        if (neg)
            c = read();
        do {
            ret = ret * 10 + c - '0';
        }
        while ((c = read()) >= '0' && c <= '9');
        if (neg)
            return -ret;
        return ret;
    }

    public double nextDouble() throws IOException {
        double ret = 0, div = 1;
        byte c = read();
        while (c <= ' ')
            c = read();
        boolean neg = (c == '-');
        if (neg)
            c = read();

        do {
            ret = ret * 10 + c - '0';
        }
        while ((c = read()) >= '0' && c <= '9');

        if (c == '.') {
            while ((c = read()) >= '0' && c <= '9') {
                ret += (c - '0') / (div *= 10);
            }
        }

        if (neg)
            return -ret;
        return ret;
    }

    private void fillBuffer() throws IOException {
        bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
        if (bytesRead == -1)
            buffer[0] = -1;
    }

    private byte read() throws IOException {
        if (bufferPointer == bytesRead)
            fillBuffer();
        return buffer[bufferPointer++];
    }

    public void close() throws IOException {
        if (din == null)
            return;
        din.close();
    }
}
