#include <bits/stdc++.h>
using namespace std;
template <class TAT>
inline void read(TAT &a) {
  static char cc;
  static bool f;
  while ((cc = getchar()) != '-' && (cc < '0' || cc > '9'))
    ;
  if (cc == '-')
    f = 1, a = 0;
  else
    f = 0, a = cc - '0';
  while ((cc = getchar()) >= '0' && cc <= '9') a = a * 10 + cc - '0';
  if (f) a = -a;
}
template <class TAT>
inline void write(TAT a) {
  static char cc[27];
  static int ct;
  if (a == 0) {
    putchar('0');
    return;
  }
  if (a < 0) {
    a = -a;
    putchar('-');
  }
  ct = 0;
  while (a) cc[++ct] = a % 10 + '0', a /= 10;
  while (ct) putchar(cc[ct--]);
}
template <class TAT>
inline void Ckmin(TAT &a, const TAT &b) {
  if (a > b) a = b;
}
template <class TAT>
inline void Ckmax(TAT &a, const TAT &b) {
  if (a < b) a = b;
}
void begin() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
void end() {
  fclose(stdin);
  fclose(stdout);
}
const int maxn = 1e5 + 115;
int n;
char S[maxn];
char C[2] = {'b', 'r'};
int main() {
  int Ans = 2e9, Cnt[2], nt;
  read(n);
  scanf("%s", S + 1);
  nt = 0;
  Cnt[0] = Cnt[1] = 0;
  for (int i = 1; i <= n; ++i) {
    Cnt[nt] += (S[i] != C[nt]);
    nt ^= 1;
  }
  Ckmin(Ans, max(Cnt[0], Cnt[1]));
  nt = 1;
  Cnt[0] = Cnt[1] = 0;
  for (int i = 1; i <= n; ++i) {
    Cnt[nt] += (S[i] != C[nt]);
    nt ^= 1;
  }
  Ckmin(Ans, max(Cnt[0], Cnt[1]));
  write(Ans);
  putchar('\n');
  return 0;
}
