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

#include <bits/stdc++.h>
using namespace std;
template <typename T>
T gcd(T a, T b) {
  return a ? gcd(b % a, a) : b;
}
template <typename T>
T lcm(T a, T b) {
  return (a / gcd(a, b)) * b;
}
string int_to_str_easy(long long n) {
  return n ? "" : int_to_str_easy(n / 10) + (char)('0' + n % 10);
}
template <typename T>
T sum(vector<T> v) {
  T ans = 0;
  for (T i : v) ans += i;
  return ans;
}
bool is_down(char x) { return ('a' <= x && x <= 'z'); }
bool is_upper(char x) { return ('A' <= x && x <= 'Z'); }
bool is_digit(char x) { return ('0' <= x && x <= '9'); }
string follow(string s) {
  for (int i = 0; i < s.length(); i++)
    if (is_upper(s[i])) s[i] += 'a' - 'A';
  return s;
}
const long double pi = 3.141592653589793238462643383279;
const long double log23 = 1.58496250072115618145373894394781;
const long double eps = 1e-8;
const long double zero = 0;
const long long INF = 1e18;
const int COUT = 30;
const int prost = 239;
const long long prost64 = 239;
const int two = 2;
const long long MOD = 1000000007;
const int BIG = 1e9;
const int alf = 26;
const int MAX_M = 2001;
const int MAX_N = 2 * 1e5 + 1;
const int km = (1 << 18) - 1;
const int dxo[8] = {-1, -1, -1, 0, 1, 1, 1, 0};
const int dyo[8] = {-1, 0, 1, 1, 1, 0, -1, -1};
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, 1, 0, -1};
const int bit_max = 32;
void solve() { return; }
int n;
string s;
int main() {
  cin.sync_with_stdio(0);
  cin >> n >> s;
  string e = "";
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0)
      e += 'r';
    else
      e += 'b';
  }
  int now = 0;
  string w = s;
  int no1 = 0;
  int no2 = 0;
  for (int i = 0; i < n; i++) {
    if (w[i] != e[i] && (i + 1) % 2 == 0)
      no1++;
    else if (w[i] != e[i])
      no2++;
  }
  now = min(no1, no2) + abs(no1 - no2);
  ans = now;
  now = 0;
  e = "";
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0)
      e += 'b';
    else
      e += 'r';
  }
  w = s;
  no1 = 0;
  no2 = 0;
  for (int i = 0; i < n; i++) {
    if (w[i] != e[i] && (i + 1) % 2 == 0)
      no1++;
    else if (w[i] != e[i])
      no2++;
  }
  now = min(no1, no2) + abs(no1 - no2);
  ans = min(ans, now);
  cout << ans;
  return 0;
}
