#include <bits/stdc++.h>
using namespace std;
template <class S, class T>
pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first + t.first, s.second + t.second);
}
template <class S, class T>
pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first - t.first, s.second - t.second);
}
template <class S, class T>
ostream &operator<<(ostream &os, pair<S, T> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long inf = 1000000001;
const long long INF = 2e18;
const long long MOD = 1000000007;
const double pi = 3.14159265358979323846;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
int dx2[8] = {1, 1, 0, -1, -1, -1, 0, 1}, dy2[8] = {0, 1, 1, 1, 0, -1, -1, -1};
vector<vector<vector<long long> > > dp(
    2, vector<vector<long long> >(5020, vector<long long>(3, INF)));
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  s.push_back('0');
  vector<string> t;
  string now;
  for (int i = 0; i < n; i++) {
    if (s[i] == '1') {
      now.push_back('1');
    } else {
      if (s[i + 1] == '1') {
        if (now.size() > 0) {
          now.push_back('0');
        }
      } else {
        if (now.size() > 0) {
          t.push_back(now);
        }
        now.clear();
      }
    }
  }
  if (now.size() > 0) {
    t.push_back(now);
  }
  int ans = 0;
  for (int q = 0; q < t.size(); q++) {
    s = t[q];
    vector<int> a;
    int cnt = 0;
    for (int i = 0; i < s.size(); i++) {
      if (s[i] == '1') {
        cnt++;
      } else {
        a.push_back(cnt);
        cnt = 0;
      }
    }
    a.push_back(cnt);
    vector<vector<int> > dp(a.size(), vector<int>(4, -inf));
    dp[0][3] = 0;
    for (int i = 0; i < a.size() - 1; i++) {
      for (int j = 0; j < 4; j++) {
        if (dp[i][j] <= -inf) {
          continue;
        }
        if (j == 0) {
          chmax(dp[i + 1][0], dp[i][0] + a[i] - 1);
          chmax(dp[i + 1][3], dp[i][0]);
          if (a[i] - 1 > 0) {
            chmax(dp[i + 1][1], dp[i][0] + a[i + 1]);
            chmax(dp[i + 1][2], dp[i][0] + a[i + 1] - 1);
          }
        } else if (j == 1) {
          chmax(dp[i + 1][3], dp[i][1]);
        } else if (j == 2) {
          chmax(dp[i + 1][3], dp[i][2]);
          chmax(dp[i + 1][1], dp[i][2] + a[i + 1]);
          chmax(dp[i + 1][2], dp[i][2] + a[i + 1] - 1);
        } else {
          chmax(dp[i + 1][0], dp[i][3] + a[i]);
          chmax(dp[i + 1][3], dp[i][3]);
          chmax(dp[i + 1][1], dp[i][3] + a[i + 1]);
          chmax(dp[i + 1][2], dp[i][3] + a[i + 1] - 1);
        }
      }
    }
    int res = 0;
    for (int j = 0; j < 4; j++) {
      res = max(res, dp[a.size() - 1][j]);
    }
    ans += res;
  }
  cout << ans << endl;
}
