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.io.*;
public class a{

    public static void main(final String [] args) throws IOException{
       BufferInput sc=new BufferInput();
       int n=sc.nextInt();
       String s=sc.next();// r-1,b=0
       int [] a=new int[n];
       for(int i=0;i<n;i++)
         a[i]=s.charAt(i)=='r'?1:0;
       
       int [] x=new int[n];
       int [] y=new int[n];
       x[0]=0;
       y[0]=1;
       for(int i=1;i<n;i++){
         x[i]=1-x[i-1];
         y[i]=1-y[i-1];
       }

       int ans=Integer.MAX_VALUE;
       int zero=0,one=0;

       for(int i=0;i<n;i++){
         if(a[i]!=x[i]){
           if(a[i]==0)zero++;
           else one++;
         }
       }

       ans=Math.min(ans,one+zero-Math.min(one,zero));

       zero=0;one=0;
       for(int i=0;i<n;i++){
        if(a[i]!=y[i]){
          if(a[i]==0)zero++;
          else one++;
        }
      }

      ans=Math.min(ans,one+zero-Math.min(one,zero));

      System.out.println(ans);


       





    }
  }


































































































  
 
 
class BufferInput 
 
{	
	final private int BUFFER_SIZE = 1 << 16;
	
	private DataInputStream din;
	
	private byte[] buffer;
	
	private int bufferPointer, bytesRead;
	
	public BufferInput() {
		din = new DataInputStream(System.in);
		buffer = new byte[BUFFER_SIZE];
		bufferPointer = bytesRead = 0;
	}
 
	public BufferInput(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[64];
		int cnt = 0, c;
		while ((c = read()) != -1) {
			if (c == '\n')
				break;
			buf[cnt++] = (byte) c;
		}
		return new String(buf, 0, cnt);
	}
    
    public String next() throws IOException{
 
		byte c = read();
		while(Character.isWhitespace(c)){
			c = read();
		}
		
		StringBuilder builder = new StringBuilder();
		builder.append((char)c);
		c = read();
		while(!Character.isWhitespace(c)){
			builder.append((char)c);
			c = read();
		}
		
		return builder.toString();
	}
 
	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 int[] nextIntArray(int n) throws IOException {
		int arr[] = new int[n];
		for(int i = 0; i < n; i++){
			arr[i] = nextInt();
		}
		return arr;
	}
 
	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 long[] nextLongArray(int n) throws IOException {
		long arr[] = new long[n];
		for(int i = 0; i < n; i++){
			arr[i] = nextLong();
		}
		return arr;
	}
 
	public char nextChar() throws IOException{
		byte c = read();
		while(Character.isWhitespace(c)){
			c = read();
		}
		return (char) c;	
	}
	
	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;
    }
    
	public double[] nextDoubleArray(int n) throws IOException {
		double arr[] = new double[n];
		for(int i = 0; i < n; i++){
			arr[i] = nextDouble();
		}
		return arr;
	}
 
	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();
	}
 
}