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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Collections;
import java.util.StringTokenizer;

/**
 * Created by yujiahao on 4/18/17.
 */
public class cf_373_b {
    private FastScanner in;
    private PrintWriter out;

    public void solve() throws IOException {
        int n = in.nextInt();
        char[] cs = in.next().toCharArray();
        int[] nums = new int[n];
        for (int i=0; i<n; i++){
            if (cs[i] == 'r')   nums[i] = 0;
            else nums[i] = 1;
        }

        int[] cnt = new int[2];

        for (int i=1; i<n; i++){
            int pre = nums[i-1];
            int cur = nums[i];
            if (cur!=pre)   continue;

            cnt[cur]++;
            nums[i] = 1-cur;
        }

        out.println(Math.abs(cnt[0] - cnt[1]) + Math.min(cnt[0], cnt[1]));

    }

    public void run() {
        try {
            in = new FastScanner();
            out = new PrintWriter(System.out);
            solve();
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private class FastScanner {
        private BufferedReader br;
        private StringTokenizer st;

        public FastScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

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

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

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

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

        public BigInteger nextBigInteger() { return new BigInteger(next());}

    }

    public static void main(String[] arg) {
        new cf_373_b().run();
    }
}
