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 MainAlter {

	public static void main(String[] args) {
		FasterScanner in=new FasterScanner();
		int n=in.nextInt();
		String data=in.nextString();
		int bcount=0,rcount=0;
		int i,j;
		for(i=0;i<n;i++)
		{
			if(data.charAt(i)=='b') bcount++;
			else rcount++;
		}
		if(bcount==rcount)
		{
			//consider rbrbrbrbbr
			int dis=0;
			for(i=0;i<n;i+=2)
			{
				if(data.charAt(i)=='b') dis++;
			}
			for(i=1;i<n;i+=2)
			{
				if(data.charAt(i)=='r') dis++;
			}
			int min=dis;
			dis=0;
			for(i=0;i<n;i+=2)
			{
				if(data.charAt(i)=='r') dis++;
			}
			for(i=1;i<n;i+=2)
			{
				if(data.charAt(i)=='b') dis++;
			}
			min=Math.min(min, dis);
			System.out.println(min/2);
			
		}
		
		else
		{
			
			int half=(int)Math.ceil(n*1.0/2.0);
			//consider brbrbrbrb
			int change=Math.abs(bcount-half);
			int dis=0;
			for(i=0;i<n;i+=2)
			{
				if(data.charAt(i)=='r') dis++;
			}
			for(i=1;i<n;i+=2)
			{
				if(data.charAt(i)=='b') dis++;
			}
			int min=(dis-change)/2+change;
			change=Math.abs(rcount-half);
			dis=0;
			for(i=0;i<n;i+=2)
			{
				if(data.charAt(i)=='b') dis++;
			}
			for(i=1;i<n;i+=2)
			{
				if(data.charAt(i)=='r') dis++;
			}
			min=Math.min(min, ((dis-change)/2)+change);
			System.out.println(min);
		}
			
		// TODO Auto-generated method stub

	}

}

class FasterScanner {
	 
    private byte[] buf = new byte[8192];
    private int curChar;
    private int numChars;

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

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

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

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

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

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

    public long[] nextLongArray(int n) {
        long[] arr = new long[n];
        for (int i = 0; i < n; i++) {
            arr[i] = nextLong();
        }
        return arr;
    }

    private boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

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