#include <bits/stdc++.h>
using namespace std;
inline int getc() {
  char ch = getchar();
  while (ch != '0' && ch != '1') {
    ch = getchar();
  }
  return ch - '0';
}
int main() {
  int n;
  scanf("%d", &n);
  int cnt1 = 0, cnt2 = 0;
  for (int i = 0; i < n; ++i) {
    if (getc() == i % 2)
      cnt1++;
    else
      cnt2++;
  }
  printf("%d\n", min(cnt1, cnt2));
  ;
  ;
  ;
}
