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 <class T, class U>
inline void relax_min(T &a, U b) {
  a = min(a, b);
}
template <class T, class U>
inline void relax_max(T &a, U b) {
  a = max(a, b);
}
template <typename T>
inline bool updmax(T &a, const T &b) {
  return a < b ? a = b, 1 : 0;
}
template <typename T>
inline bool updmin(T &a, const T &b) {
  return a > b ? a = b, 1 : 0;
}
template <class T, class U>
istream &operator>>(istream &in, pair<T, U> &a) {
  in >> a.first >> a.second;
  return in;
}
template <class T, class U>
ostream &operator<<(ostream &out, const pair<T, U> &a) {
  out << a.first << ' ' << a.second;
  return out;
}
template <class T>
istream &operator>>(istream &in, vector<T> &a) {
  for (auto((i)) = (0); ((i)) < ((((int)(a).size()))); ++((i))) in >> a[i];
  return in;
}
template <class T>
ostream &operator<<(ostream &out, const vector<T> &a) {
  for (auto((i)) = (0); ((i)) < ((((int)(a).size()))); ++((i)))
    out << a[i] << ' ';
  return out;
}
int next_int() {
  int(tmp);
  scanf("%d", &(tmp));
  return tmp;
}
long long modPow(long long a, long long b, long long MOD) {
  long long ret = 1;
  for (; b; b >>= 1) {
    if (b & 1)
      ret = ((((((long long)(ret) * (long long)(a))) % ((MOD))) + ((MOD))) %
             ((MOD)));
    a = ((((((long long)(a) * (long long)(a))) % ((MOD))) + ((MOD))) % ((MOD)));
  }
  return ret;
}
const int MAXN1 = (int)1e5 + 3;
const int MAXN2 = (int)2e5 + 3;
const int MAXN3 = (int)3e5 + 3;
const int MAXN4 = (int)4e5 + 3;
const int MAXN5 = (int)5e5 + 3;
const int INF = (int)1e9 + 7;
const int INF2 = 1 << 30;
const long long INFLL = (long long)4e18 + 7;
const long long INFLL2 = 1LL << 60;
const long double Pi = acos(-1.0);
const long double eps = 1e-7;
const int mod = (int)1e9 + 7;
int main() {
  int(n);
  scanf("%d", &(n));
  string str;
  cin >> str;
  string a = str;
  int r = (n + 1) / 2;
  int b = n / 2;
  int cntr = 0, cntb = 0;
  for (auto((i)) = (0); ((i)) < ((n)); ++((i)))
    if (str[i] == 'r')
      ++cntr;
    else
      ++cntb;
  int ans = abs(cntr - r);
  int delr = r - cntr, delb = b - cntb;
  for (auto((i)) = (0); ((i)) < ((n)); ++((i))) {
    if (delr > delb && a[i] == 'b' && i % 2 == 0 && delr > 0) {
      a[i] = 'r';
      --delr;
    } else if (delr < delb && a[i] == 'r' && i % 2 == 1 && delb > 0) {
      a[i] = 'b';
      --delb;
    }
  }
  if (delr > delb && delr > 0) {
    for (auto((i)) = (0); ((i)) < ((n)); ++((i))) {
      if (cntb <= 0) break;
      if (a[i] == 'b') {
        a[i] = 'r';
        --delr;
      }
    }
  }
  if (delb > delr && delb > 0) {
    for (auto((i)) = (0); ((i)) < ((n)); ++((i))) {
      if (cntr <= 0) break;
      if (a[i] == 'r') {
        a[i] = 'b';
        --delb;
      }
    }
  }
  int cnt = 0;
  for (auto((i)) = (0); ((i)) < ((n)); ++((i)))
    if ((a[i] == 'r' && i % 2 == 1) || (a[i] == 'b' && i % 2 == 0)) ++cnt;
  ans += cnt / 2;
  a = str;
  b = (n + 1) / 2;
  r = n / 2;
  cntr = 0, cntb = 0;
  for (auto((i)) = (0); ((i)) < ((n)); ++((i)))
    if (str[i] == 'r')
      ++cntr;
    else
      ++cntb;
  int ans2 = abs(cntr - r);
  delr = r - cntr, delb = b - cntb;
  for (auto((i)) = (0); ((i)) < ((n)); ++((i))) {
    if (delr > delb && a[i] == 'b' && i % 2 == 1 && cntb > 0) {
      a[i] = 'r';
      --cntb;
    } else if (delr < delb && a[i] == 'r' && i % 2 == 0 && cntr > 0) {
      a[i] = 'b';
      --cntr;
    }
  }
  if (delr > delb && delr > 0) {
    for (auto((i)) = (0); ((i)) < ((n)); ++((i))) {
      if (cntb <= 0) break;
      if (a[i] == 'b') {
        a[i] = 'r';
        --delr;
      }
    }
  }
  if (delb > delr && delb > 0) {
    for (auto((i)) = (0); ((i)) < ((n)); ++((i))) {
      if (cntr <= 0) break;
      if (a[i] == 'r') {
        a[i] = 'b';
        --delb;
      }
    }
  }
  cnt = 0;
  for (auto((i)) = (0); ((i)) < ((n)); ++((i)))
    if ((a[i] == 'r' && i % 2 == 0) || (a[i] == 'b' && i % 2 == 1)) ++cnt;
  ans2 += cnt / 2;
  cout << min(ans, ans2) << endl;
  return 0;
}
