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.util.*;

public class main {

    public static void main(String[] args) {    
        InputReader sc=new InputReader(System.in);
        int n=sc.nextInt();
        String str =sc.nextLine();
        int b=0,r=0,bo=0,be=0,ro=0,re=0;
        for(int i=0;i<str.length();i++){
        	if(str.charAt(i)=='b'){
        		b++;
        		if(i%2==0)
        			be++;
        		else
       				bo++;
        	}
        	else{ 
        		r++;
        		if(i%2==0)
        			re++;
        		else
        			ro++;
        	}
        }
        if(ro+be >= bo+re){
        	System.out.println(Math.max(bo, re));
        }
        else
        	System.out.println(Math.max(ro, be));
    }
    
    static class Pair implements Comparable<Pair> {
		int idx;
		double val;

		Pair(int i, double v) {
			idx = i;
			val = v;
		}

		public int compareTo(Pair o) {
			if (val != o.val)
				return Double.compare(val, o.val);
			return Integer.compare(idx, o.idx);
		}

		public String toString() {
			return idx + " " + val;
		}
	}

    
    static class InputReader {

    private final InputStream stream;
    private final byte[] buf = new byte[8192];
    private int curChar, snumChars;
    private SpaceCharFilter filter;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int snext() {
        if (snumChars == -1)
            throw new InputMismatchException();
        if (curChar >= snumChars) {
            curChar = 0;
            try {
                snumChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (snumChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public int nextInt() {
        int c = snext();
        while (isSpaceChar(c)) {
            c = snext();
        }
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = snext();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = snext();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public long nextLong() {
        int c = snext();
        while (isSpaceChar(c)) {
            c = snext();
        }
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = snext();
        }
        long res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = snext();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public int[] nextIntArray(int n) {
        int a[] = new int[n];
        for (int i = 0; i < n; i++) {
            a[i] = nextInt();
        }
        return a;
    }

    public String readString() {
        int c = snext();
        while (isSpaceChar(c)) {
            c = snext();
        }
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = snext();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public String nextLine() {
        int c = snext();
        while (isSpaceChar(c))
            c = snext();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = snext();
        } while (!isEndOfLine(c));
        return res.toString();
    }

    public boolean isSpaceChar(int c) {
        if (filter != null)
            return filter.isSpaceChar(c);
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    private boolean isEndOfLine(int c) {
        return c == '\n' || c == '\r' || c == -1;
    }

    public interface SpaceCharFilter {
        public boolean isSpaceChar(int ch);
    }
}
}