#include <bits/stdc++.h>
using namespace std;
string s;
int n;
int oddb, evenb, oddr, evenr;
int main() {
  cin >> n;
  cin >> s;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0) {
      if (s[i] == 'b')
        evenb++;
      else if (s[i] == 'r')
        evenr++;
    } else if (i % 2 == 1) {
      if (s[i] == 'r')
        oddr++;
      else if (s[i] == 'b')
        oddb++;
    }
  }
  ans = min(max(oddr, evenb), max(oddb, evenr));
  cout << ans;
  return 0;
}
