#include <bits/stdc++.h>
using namespace std;
const int INF = 0x7f7f7f7f;
char s[1005];
int dp[1005][2];
int main() {
  int n;
  scanf("%d", &n);
  scanf("%s", s);
  if (1 == n) {
    puts("0");
    return 0;
  }
  memset(dp, 0x7f, sizeof(dp));
  dp[0][s[0] - '0'] = 0;
  dp[0][(s[0] - '0') ^ 1] = 1;
  for (int i = 1; i < n; ++i) {
    if (s[i] == '1') {
      dp[i][1] = dp[i - 1][0];
      dp[i][0] = dp[i - 1][1] + 1;
    } else {
      dp[i][0] = dp[i - 1][1];
      dp[i][1] = dp[i - 1][0] + 1;
    }
  }
  int ans = min(dp[n - 1][0], dp[n - 1][1]);
  if (ans >= INF) ans = -1;
  printf("%d\n", ans);
  return 0;
}
