#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  char m = 'b';
  int ans = 0;
  int k = 0, bm = 0, rm = 0;
  while (k < n) {
    if (s[k] == m) {
      if (m == 'b')
        m = 'r';
      else
        m = 'b';
      k++;
    } else {
      if (s[k] == 'b')
        bm++;
      else
        rm++;
      if (m == 'b')
        m = 'r';
      else
        m = 'b';
      k++;
    }
  }
  int ans1 = min(bm, rm) + abs(rm - bm);
  m = 'r';
  k = 0, bm = 0, rm = 0;
  while (k < n) {
    if (s[k] == m) {
      if (m == 'b')
        m = 'r';
      else
        m = 'b';
      k++;
    } else {
      if (s[k] == 'b')
        bm++;
      else
        rm++;
      if (m == 'b')
        m = 'r';
      else
        m = 'b';
      k++;
    }
  }
  int ans2 = min(bm, rm) + abs(rm - bm);
  cout << min(ans1, ans2) << endl;
  return 0;
}
