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.lang.*;
import java.io.*;

public class B_373
{
	public static void main (String[] args) throws java.lang.Exception
	{
	
	//	Scanner scan=new Scanner(System.in);
		InputReader in =new InputReader(System.in);
		PrintWriter out=new PrintWriter(System.out);
//		int t=in.nextInt();
 		int t=1;
		while(t-->0){
		int n=in.nextInt();
		String str=in.next();
		int cntr=0,cntb=0;
		for(int i=0;i<n;i=i+2){
			if(str.charAt(i)!='r')cntr++;
			if(i+1<n&&str.charAt(i+1)!='b')cntb++;
		}
		int min=Math.min(cntr,cntb);
		int ans1=0;
		ans1+=cntr-min;
		ans1+=cntb-min;
		ans1+=min;
		
		cntr=0;cntb=0;
		for(int i=0;i<n;i=i+2){
			if(str.charAt(i)!='b')cntr++;
			if(i+1<n&&str.charAt(i+1)!='r')cntb++;
		}
		min=Math.min(cntr,cntb);
		int ans2=0;
		ans2+=cntr-min;
		ans2+=cntb-min;
		ans2+=min;
		out.println(Math.min(ans1,ans2));
	}
	out.close();
	}
	
	
	static class InputReader
	{
	 
	    private InputStream stream;
	    private byte[] buf = new byte[1024];
	    private int curChar;
	    private int numChars;
	 
	    public InputReader(InputStream stream) {
	        this.stream = stream;
	    }
	 
	    public int read() {
	        if (numChars == -1)
	            throw new InputMismatchException();
	        if (curChar >= numChars) {
	            curChar = 0;
	            try {
	                numChars = stream.read(buf);
	            } catch (IOException e) {
	                throw new InputMismatchException();
	            }
	            if (numChars <= 0)
	                return -1;
	        }
	        return buf[curChar++];
	    }
	 
	    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 & 15;
	            c = read();
	        } while (!isSpaceChar(c));
	        return res * sgn;
	    }
	 
	    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 & 15;
	            c = read();
	        } while (!isSpaceChar(c));
	        return res * sgn;
	    }
	 
	    public String next() {
	        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 String nextLine() {
	        int c = read();
	        //while (c != '\n' && c != '\r' && c != '\t' && c != -1)
	        //c = read();
	        StringBuilder res = new StringBuilder();
	        do {
	            res.appendCodePoint(c);
	            c = read();
	        } while (c != '\n' && c != '\r' && c != '\t' && c != -1);
	        return res.toString();
	    }
	 
	    public static boolean isSpaceChar(int c) {
	        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
	    }
	 
	}   

}






