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 TA, typename TB>
ostream &operator<<(ostream &ostm, const pair<TA, TB> &p) {
  ostm << "(" << p.first << "," << p.second << ")";
  return ostm;
}
template <typename T>
ostream &operator<<(ostream &ostm, const vector<T> &v) {
  ostm << "[ ";
  for (auto i : v) ostm << i << " ";
  ostm << "]";
  return ostm;
}
template <typename TA, typename TB>
ostream &operator<<(ostream &ostm, const map<TA, TB> &mp) {
  ostm << "[ ";
  for (auto it : mp) ostm << *it << " ";
  ostm << "]";
  return ostm;
}
template <typename T>
ostream &operator<<(ostream &ostm, const set<T> &s) {
  ostm << "[ ";
  for (auto it : s) ostm << *it << " ";
  ostm << "]";
  return ostm;
}
template <typename T>
ostream &operator<<(ostream &ostm, const stack<T> &inp) {
  stack<T> st = inp;
  ostm << "[ ";
  while (!st.empty()) {
    ostm << st.top() << " ";
    st.pop();
  }
  ostm << "]";
  return ostm;
}
template <typename T>
ostream &operator<<(ostream &ostm, const queue<T> &inp) {
  queue<T> q = inp;
  ostm << "[ ";
  while (!q.empty()) {
    ostm << q.front() << " ";
    q.pop();
  }
  ostm << "]";
  return ostm;
}
template <typename T>
ostream &operator<<(ostream &ostm, const priority_queue<T> &inp) {
  priority_queue<T> pq = inp;
  ostm << "[ ";
  while (!pq.empty()) {
    ostm << pq.top() << " ";
    pq.pop();
  }
  ostm << "]";
  return ostm;
}
template <typename T>
ostream &operator<<(ostream &ostm, const deque<T> &inp) {
  deque<T> dq = inp;
  ostm << "[ ";
  while (!dq.empty()) {
    ostm << dq.front() << " ";
    dq.pop_front();
  }
  ostm << "]";
  return ostm;
}
inline int lowbit(int &x) { return x & -x; }
inline long long lowbit(long long &x) { return x & -x; }
template <typename T>
inline T _pow(T b, int n) {
  T a = 1;
  while (n) {
    if (n & 1) a *= b;
    b *= b, n >>= 1;
  }
  return a;
}
template <typename T>
inline T _pow(T b, int n, T mod) {
  T a = 1 % mod;
  while (n) {
    if (n & 1) a = a * b % mod;
    b = b * b % mod, n >>= 1;
  }
  return a;
}
inline int gtx() {
  const int N = 1048576;
  static char __buffer[N];
  static char *__p = __buffer, *__end = __buffer;
  if (__p == __end) {
    if ((__end = __buffer + fread(__buffer, 1, N, stdin)) == __buffer)
      return EOF;
    __p = __buffer;
  }
  return *__p++;
}
template <typename T>
inline bool rit(T &x) {
  char c = 0;
  bool fg = 0;
  while (c = gtx(), (c < '0' && c != '-') || c > '9')
    if (c == EOF) return false;
  c == '-' ? (fg = 1, x = 0) : (x = c - '0');
  while (c = gtx(), c >= '0' && c <= '9') x = x * 10 + c - '0';
  if (fg) x = -x;
  return true;
}
template <typename T, typename... Args>
inline bool rit(T &x, Args &...args) {
  return rit(x) && rit(args...);
}
inline void pit(int x) { printf("%d", x); }
inline void pln(long long x) { printf("%I64d", x); }
template <typename... Args>
inline void pit(int x, Args... args) {
  printf("%d ", x);
  pit(args...);
}
template <typename... Args>
inline void pln(long long x, Args... args) {
  printf("%I64d ", x);
  pit(args...);
}
const long double PI = 3.14159265358979323846264338327950288;
const long double eps = 1e-8;
const long long mod = 1e9 + 7;
int startB(int n, int b, int r, string &s) {
  int ans = 0;
  int cb = 0, cr = 0;
  for (int i = 0; i < n; ++i) {
    if (i % 2 == 0 && s[i] != 'b') {
      ++ans;
      ++cb;
    }
    if (i % 2 && s[i] != 'r') {
      ++ans;
      ++cr;
    }
  }
  return ans - min(cb, cr);
}
int startR(int n, int b, int r, string &s) {
  int ans = 0;
  int cb = 0, cr = 0;
  for (int i = 0; i < n; ++i) {
    if (i % 2 == 0 && s[i] != 'r') {
      ++ans;
      ++cb;
    }
    if (i % 2 && s[i] != 'b') {
      ++ans;
      ++cr;
    }
  }
  return ans - min(cb, cr);
}
int main() {
  int n;
  string s;
  cin >> n >> s;
  int b = 0, r = 0;
  for (char c : s) {
    if (c == 'b')
      ++b;
    else
      ++r;
  }
  int ans = min({startB(n, b, r, s), startR(n, b, r, s)});
printAns:;
  cout << ans << '\n';
}
