#include <bits/stdc++.h>
using namespace std;

// template {{{
#define  range(i, l, r) for (int i = (int)(l); i < (int)(r); (i) += 1)
#define rrange(i, l, r) for (int i = (int)(r) - 1; i >= (int)(l); (i) -= 1)

#define  whole(f, x, ...) ([&](decltype((x)) container) { return (f)(  begin(container),  end(container), ## __VA_ARGS__); })(x)
#define rwhole(f, x, ...) ([&](decltype((x)) container) { return (f)( rbegin(container), rend(container), ## __VA_ARGS__); })(x)

#define debug(x) cerr << "(" << __LINE__ << ")" << #x << ": " << (x) << endl

using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;

// constexpr i32 mod   = 998244353;
constexpr i32 mod   = 1e9 + 7;
constexpr i32 inf   = 1001001001;
constexpr i64 infll = 1001001001001001001ll;

constexpr int dx[] = {0, -1, 1, 0, -1, 1, -1, 1};
constexpr int dy[] = {-1, 0, 0, 1, -1, -1, 1, 1};

struct IoSetup { IoSetup(int x = 15){ cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(x); cerr << fixed << setprecision(x); } } iosetup;

template <typename T = i64> T input() { T x; cin >> x; return x; }

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) { range(i, 0, v.size()) { os << v[i] << (i + 1 != v.size() ? " " : ""); } return os; }
template <typename T> istream &operator>>(istream &is, vector<T> &v) { for (T &in : v) is >> in; return is; }

template <typename T1, typename T2> ostream &operator<<(ostream &os, pair<T1, T2> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
template <typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }

template <typename T> vector<T> make_vector(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_vector(size_t a, Ts... ts) { return vector<decltype(make_vector(ts...))>(a, make_vector(ts...)); }

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
// }}}

void solver() {
    int n = input();
    string s;
    cin >> s;

    s = '0' + s + '0';

    vector< pair< int, int > > vs;
    range(i, 1, n + 1) {
        if (s[i] == '1') continue;
        if (s[i - 1] == '0' || s[i + 1] == '0') continue;

        int l = i - 1;
        int r = i + 1;
        while (s[l] == '1') l--;
        while (s[r] == '1') r++;

        vs.emplace_back(i - 1, r - 1);
        vs.emplace_back(l + 1, i + 1);
        if (s[r - 2] == '1') {
            vs.emplace_back(i - 1, r - 2);
        }
        if (s[l + 2] == '1') {
            vs.emplace_back(l + 2, i + 1);
        }
    }

    if (vs.size() == 0) {
        cout << 0 << endl;
        return;
    }

    whole(sort, vs);
    vs.erase(whole(unique, vs), vs.end());

    vector< int > dp(vs.size());
    range(i, 0, vs.size()) {
        int op = vs[i].second - vs[i].first - 1;
        dp[i] = op;
        rrange(j, max(0, i - 100), i) {
            if (vs[j].second < vs[i].first) {
                chmax(dp[i], dp[j] + op);
            }
        }
    }

    cout << *whole(max_element, dp) << endl;
}

signed main(int argc, char *argv[]) {
    solver();
}


Measured memory_bytes: 3888