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.Scanner;

public class AnatolyAndCockroaches {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		char[] c = sc.next().toCharArray();
		int ans = Integer.MAX_VALUE, r = 0, b = 0;
		for (int i = 0; i < n; i++)
			if (c[i] == 'r' && (i & 1) != 0)
				r++;
			else if (c[i] == 'b' && (i & 1) == 0)
				b++;
		ans = Math.min(ans, Math.max(r, b));
		r = b = 0;
		for(int i= 0;i < n;i++)
			if (c[i] == 'r' && (i & 1) == 0)
				r++;
			else if (c[i] == 'b' && (i & 1) != 0)
				b++;
		ans = Math.min(ans, Math.max(r, b));
		System.out.println(ans);
	}

}