codef

#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<map>
using namespace std;
#define boostio ios_base::sync_with_stdio(0);
#define ll long long
#define MAX 30009

int main() {
    boostio;
    string orig;
    int n, bs1, bs2, rs1, rs2, c1, c2;
    cin >> n;
    string s1, s2, new1, new2;

    cin >> orig;

    for(int i = 0; i<n; i++){
    	if(i%2 == 0){s1[i] = 'b'; s2[i] = 'r';}
    	if(i%2 == 1){s1[i] = 'r'; s2[i] = 'b';}
    }

	for(int i = 0; i<n; i++){
    	if(orig[i] != s1[i]){new1.push_back(s1[i]);}
    	if(orig[i] != s2[i]){new2.push_back(s2[i]);}
    }

	bs1 = count(new1.begin(), new1.end(), 'b');
	rs1 = count(new1.begin(), new1.end(), 'r');
	bs2 = count(new2.begin(), new2.end(), 'b');
	rs2 = count(new2.begin(), new2.end(), 'r');

	c1 = max(bs1,rs1);
	c2 = max(bs2,rs2);

	cout << min(c1,c2);

    return 0;
}