#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void rread(T& num) {
  num = 0;
  T f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') num = num * 10 + ch - '0', ch = getchar();
  num *= f;
}
const long long inf = 1e16;
const int maxn = 1e5 + 10, mod = 1e9 + 7;
const long double PI = acos(-1.0);
long long gcd(long long a, long long b) { return (a ? gcd(b % a, a) : b); }
void exgcd(long long a, long long b, long long& d, long long& x, long long& y) {
  if (!b) {
    d = a;
    x = 1;
    y = 0;
  } else {
    exgcd(b, a % b, d, y, x);
    y -= x * (a / b);
  }
}
long long power(long long a, long long n) {
  long long p = 1;
  while (n > 0) {
    if (n % 2) {
      p = p * a;
    }
    n >>= 1;
    a *= a;
  }
  return p;
}
long long power(long long a, long long n, long long mod) {
  long long p = 1;
  while (n > 0) {
    if (n % 2) {
      p = p * a;
      p %= mod;
    }
    n >>= 1;
    a *= a;
    a %= mod;
  }
  return p % mod;
}
int n, m;
int dp[maxn][3];
char s[maxn];
int main() {
  while (cin >> n) {
    scanf("%s", s);
    int f1 = 0, f2 = 0, ans1 = 0, ans2 = 0;
    for (int i = 0; i < n; i++) {
      if (i % 2 == 0 && s[i] == 'r') f1++;
      if (i % 2 == 1 && s[i] == 'b') f2++;
    }
    ans2 = 0;
    ans1 = max(f1, f2);
    f1 = 0;
    f2 = 0;
    for (int i = 0; i < n; i++) {
      if (i % 2 == 0 && s[i] == 'b') f1++;
      if (i % 2 == 1 && s[i] == 'r') f2++;
    }
    ans2 = max(f1, f2);
    cout << min(ans1, ans2) << endl;
  }
  return 0;
}
