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.IOException;
import java.io.InputStream;
import java.util.Arrays;
import java.util.InputMismatchException;

public class B {	
	public static void main(String[] args) {
		FastScannerB sc = new FastScannerB(System.in);
		int N = sc.nextInt();
		String s = sc.next();
		Entry[] ends_with_b = new Entry[N + 1];
		Entry[] ends_with_r = new Entry[N + 1];
		ends_with_b[0] = new Entry(0, 0, 0);
		ends_with_r[0] = new Entry(0, 0, 0);
		for(int i = 1; i <= N; i++){
			char c = s.charAt(i - 1);
			if(c == 'r'){
				ends_with_r[i] = new Entry(ends_with_b[i-1].total, ends_with_b[i-1].red_swaps, ends_with_b[i-1].black_swaps);
				ends_with_b[i] = new Entry(ends_with_r[i-1].total + 1, ends_with_r[i-1].red_swaps + 1, ends_with_r[i-1].black_swaps);
			}
			else{
				ends_with_b[i] = new Entry(ends_with_r[i-1].total, ends_with_r[i-1].red_swaps, ends_with_r[i-1].black_swaps);
				ends_with_r[i] = new Entry(ends_with_b[i-1].total + 1, ends_with_b[i-1].red_swaps, ends_with_b[i-1].black_swaps + 1);
			}
		}
		System.out.println(Math.min(Math.max(ends_with_r[N].red_swaps, ends_with_r[N].black_swaps), Math.max(ends_with_b[N].red_swaps, ends_with_b[N].black_swaps)));
	}
}

class Entry{
	int total;
	int red_swaps;
	int black_swaps;
	Entry(int t, int r, int b){
		total = t;
		red_swaps = r;
		black_swaps = b;
	}
}

class FastScannerB{
    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public FastScannerB(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 - '0';
            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();
        StringBuilder res = new StringBuilder();
        do
        {
            res.appendCodePoint(c);
            c = read();
        } while (!isLineEndChar(c));
        return res.toString();
    }    
    
    public double nextDouble() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        double res = 0;
        while (!isSpaceChar(c) && c != '.') {
            if (c == 'e' || c == 'E')
                return res * Math.pow(10, nextInt());
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        }
        if (c == '.') {
            c = read();
            double m = 1;
            while (!isSpaceChar(c)) {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, nextInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                m /= 10;
                res += (c - '0') * m;
                c = read();
            }
        }
        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 - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }
    
    public boolean isSpaceChar(int c)
    {
        if (filter != null)
            return filter.isSpaceChar(c);
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }
    
    public boolean isLineEndChar(int c)
    {
        if (filter != null)
            return filter.isSpaceChar(c);
        return c == '\n' || c == '\r' || c == -1;
    }

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