#include "bits/stdc++.h"

using namespace std;

#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl

typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
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; }
#define X first
#define Y second
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define rep1(i,n) for(ll i=1;i<=(ll)(n);i++)
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define rrep1(i,n) for(ll i=(ll)(n);i>0;i--)
#define REP(i,a,b) for(ll i=(ll)a;i<(ll)b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
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; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = 2e18;
const ll MOD = 1000000007;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
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 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);

#define vlll vector<vll>
vlll dp(2, vll(5020, vl(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);
    //DEBUG_VEC(a);
    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;
}


Measured memory_bytes: 5500