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;
int break_point() {
  char c;
  while ((c = getchar()) != '\n')
    ;
  return 0;
}
template <typename T>
void read_integer(T &r) {
  bool sign = 0;
  r = 0;
  char c;
  while (1) {
    c = getchar();
    if (c == '-') {
      sign = 1;
      break;
    }
    if (c != ' ' && c != '\n') {
      r = c - '0';
      break;
    }
  }
  while (1) {
    c = getchar();
    if (c == ' ' || c == '\n') break;
    r = r * 10 + (c - '0');
  }
  if (sign) r = -r;
}
long long binpowmod(long long a, long long b, long long mod) {
  if (b == 0) return 1;
  long long c = binpowmod(a, b >> 1, mod);
  return (((c * c) % mod) * (b & 1 ? a : 1)) % mod;
}
long long binpow(long long a, long long b) {
  if (b == 0) return 1;
  long long c = binpow(a, b >> 1);
  return c * c * (b & 1 ? a : 1);
}
inline int getbit(int x, int b) { return (x >> b) & 1; }
inline int setbit(int x, int b) { return x | (1 << b); }
inline void _setbit(int &x, int b) { x = setbit(x, b); }
inline long long setbit(long long x, int b) { return x | (1ll << b); }
inline void _setbit(long long &x, int b) { x = setbit(x, b); }
inline int unsetbit(int x, int b) { return x & (INT_MAX - (1 << b)); }
inline void _unsetbit(int &x, int b) { x = unsetbit(x, b); }
inline int countbit(int x) {
  x = x - ((x >> 1) & 0x55555555);
  x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
  return ((x + (x >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
}
inline long long countbit(long long x) {
  return countbit(int(x & INT_MAX)) + countbit(int(x >> 32) & INT_MAX);
}
inline void printbit(int x, int len) {
  for (int i = len - 1; i >= 0; i--) printf("%d", getbit(x, i));
}
int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
template <typename A, typename B>
ostream &operator<<(ostream &stream, const pair<A, B> &p) {
  stream << "{" << p.first << "," << p.second << "}";
  return stream;
}
template <typename A>
ostream &operator<<(ostream &stream, const vector<A> &v) {
  stream << "[";
  for (auto itr = v.begin(); itr != v.end(); itr++) stream << *itr << " ";
  stream << "]";
  return stream;
}
template <typename A, typename B>
ostream &operator<<(ostream &stream, const map<A, B> &v) {
  stream << "[";
  for (auto itr = v.begin(); itr != v.end(); itr++) stream << *itr << " ";
  stream << "]";
  return stream;
}
template <typename A>
ostream &operator<<(ostream &stream, const set<A> &v) {
  stream << "[";
  for (auto itr = v.begin(); itr != v.end(); itr++) stream << *itr << " ";
  stream << "]";
  return stream;
}
template <typename A>
ostream &operator<<(ostream &stream, const stack<A> &v) {
  stack<A> st = v;
  stream << "[";
  while (!st.empty()) {
    stream << st.top() << " ";
    st.pop();
  }
  stream << "]";
  return stream;
}
template <typename A>
ostream &operator<<(ostream &stream, const priority_queue<A> &v) {
  priority_queue<A> q = v;
  stream << "[";
  while (!q.empty()) {
    stream << q.top() << " ";
    q.pop();
  }
  stream << "]";
  return stream;
}
template <typename A>
ostream &operator<<(ostream &stream, const queue<A> &v) {
  queue<A> q = v;
  stream << "[";
  while (!q.empty()) {
    stream << q.front() << " ";
    q.pop();
  }
  stream << "]";
  return stream;
}
template <typename A>
ostream &operator<<(ostream &stream, const deque<A> &v) {
  deque<A> q = v;
  stream << "[";
  while (!q.empty()) {
    stream << q.front() << " ";
    q.pop_front();
  }
  stream << "]";
  return stream;
}
void run();
int main() {
  srand(time(NULL));
  run();
  return 0;
}
const int mod = 1e9 + 7;
const int N = 100005;
char s[N];
int cnt[1000];
int calc(int color, int x, int y, int n) {
  char start = color ? 'r' : 'b';
  char other = start == 'r' ? 'b' : 'r';
  int needx = n & 1 ? n / 2 + 1 : n / 2;
  int needy = n - needx;
  int x_to_y = max(0, x - needx);
  int y_to_x = max(0, y - needy);
  int wrong = 0;
  int res = 0;
  for (int i = 0; i < n; ++i) {
    if (i & 1) {
      if (s[i] != other) {
        if (x_to_y > 0) {
          --x_to_y;
          ++res;
        } else
          ++wrong;
      }
    } else {
      if (s[i] != start) {
        if (y_to_x > 0) {
          --y_to_x;
          ++res;
        } else
          ++wrong;
      }
    }
  }
  0 ? (cout << "color"
            << " = " << (color) << ","
            << "res"
            << " = " << (res) << ","
            << "wrong"
            << " = " << (wrong) << "\n")
    : cout;
  return res + wrong / 2;
}
void run() {
  int n;
  scanf("%d", &n);
  scanf("%s", s);
  for (int i = 0; i < n; ++i) ++cnt[s[i] - 'a'];
  printf("%d", min(calc(0, cnt['b' - 'a'], cnt['r' - 'a'], n),
                   calc(1, cnt['r' - 'a'], cnt['b' - 'a'], n)));
}
