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.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.util.Arrays;
import java.util.HashMap;
import java.util.InputMismatchException;
import java.util.*;

import javax.print.attribute.HashAttributeSet;
import javax.swing.plaf.synth.SynthSeparatorUI;

import org.omg.Messaging.SyncScopeHelper;

public class asd {

	public static void main(String[] args) {
		InputReader in = new InputReader(System.in);
		OutputWriter out = new OutputWriter(System.out);

		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		char[] s = scan.next().toCharArray();
		int totr = 0;
		int posr = 0;
		int posb = 0;
		int totb = 0;

		for (int i = 0; i < s.length; i = i + 2) {
			if (s[i] == 'r') {
				posr++;
			}
			if (s[i] == 'b') {
				posb++;
			}
		}
		for (int i = 0; i < s.length; i++) {
			if (s[i] == 'r') {
				totr++;
			}
			if (s[i] == 'b') {
				totb++;
			}
		}
		int as = s.length / 2;

		if (posr > posb) {
			int count = 0;
			count = count + posb;
			posr = posr + posb;
			posb = 0;
			if (as == totb) {
				System.out.println(count);
			} else {
				count = count + as - totb;
				System.out.println(count);
			}
		}

		else {
			int count = 0;
			count = count + posr;
			posb = posb + posr;
			posr = 0;
			if (as == totr) {
				System.out.println(count);
			} else {
				count = count + as - totr;
				System.out.println(count);
			}

		}

		out.close();
	}

	private 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 long readLong() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			long 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 readInt() {
			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 readString() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuffer res = new StringBuffer();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

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

		public String next() {
			return readString();
		}
	}

	private static class OutputWriter {
		private final PrintWriter writer;

		public OutputWriter(OutputStream outputStream) {
			writer = new PrintWriter(outputStream);
		}

		public OutputWriter(Writer writer) {
			this.writer = new PrintWriter(writer);
		}

		public void print(Object... objects) {
			for (int i = 0; i < objects.length; i++) {
				if (i != 0)
					writer.print(' ');
				writer.print(objects[i]);
			}
		}

		public void printLine(Object... objects) {
			print(objects);
			writer.println();
		}

		public void close() {
			writer.close();
		}
	}
}
