/*

This Submission is to determine how many 120/240 min const. delivery point there are.

//info
120 req. steps <= 5
*/
#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;
typedef string::const_iterator State;
#define Ma_PI 3.141592653589793
#define eps 1e-5
#define LONG_INF 1e18
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define MOD 1000000007
#define seg_size 262144
#define REP(a,b) for(long long a = 0;a < b;++a)

unsigned long xor128() {
	static unsigned long x = time(NULL), y = 362436069, z = 521288629, w = 88675123;
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}
double dot(complex<double> a, complex<double> b) {
	return a.real() * b.real() + a.imag() * b.imag();
}
double gyaku_dot(complex<double> a, complex<double> b) {
	return a.real() * b.imag() - a.imag() * b.real();
}
double leng(complex<double> a) {
	return sqrt(a.real() * a.real() + a.imag() * a.imag());
}
double angles(complex<double> a, complex<double> b) {
	double cosine = dot(a, b) / (leng(a) * leng(b));
	double sine = gyaku_dot(a, b) / (leng(a) * leng(b));
	double kaku = acos(min((double)1.0, max((double)-1.0, cosine)));
	if (sine <= 0) {
		kaku = 2 * Ma_PI - kaku;
	}
	return kaku;
}
vector<int> convex_hull(vector<complex<double>> a) {
	vector<int> ans;
	double now_minnest = a[0].real();
	int now_itr = 0;
	REP(i, a.size()) {
		if (now_minnest > a[i].real()) {
			now_minnest = a[i].real();
			now_itr = i;
		}
	}
	ans.push_back(now_itr);
	complex<double> ba(0, 1);
	while (true) {
		int now_go = 0;
		double now_min = 0;
		double now_length = 0;
		int starter = ans[ans.size() - 1];
		for (int i = 0; i < a.size(); ++i) {
			if (i != starter) {
				double goa = angles(ba, a[i] - a[starter]);
				if (goa - now_min >= eps || (abs(goa - now_min) <= eps && (abs(a[i] - a[starter]) - now_length) >= eps)) {
					now_min = goa;
					now_go = i;
					now_length = abs(a[i] - a[starter]);
				}
			}
		}
		if (now_go == ans[0]) break;
		ans.push_back(now_go);
		ba = complex<double>(a[now_go] - a[starter]);
	}
	return ans;
}
long long dp[4][600000] = {};
int main() {
	int n;
	cin >> n;
	n += 2;
	string s;
	cin >> s;
	s += "00";
	vector<vector<int>> inputs;
	int now = 0;
	vector<int> reject;
	REP(i, n) {
		if (s[i] == '0') {
			if (now == 0) {
				if (reject.size() >= 2) {
					inputs.push_back(reject);
				}
				reject.clear();
				now = 0;
			}
			else {
				reject.push_back(now);
				now = 0;
			}
		}
		else {
			now++;
		}
	}
	long long ans = 0;
	for (int i = 0; i < inputs.size(); ++i) {
		for (int q = 1; q < inputs[i].size(); ++q) {
			//0 -> nothing done
			//1 -> goto left
			//2 -> goto right,and stop
			//3 -> goto right,maximized
			for (int j = 0; j < 4; ++j) {
				dp[0][q + 1] = max(dp[0][q + 1], dp[j][q]);
			}
			dp[1][q + 1] = dp[0][q] + inputs[i][q - 1];
			if (inputs[i][q - 1] != 1) {
				dp[1][q + 1] = max(dp[1][q + 1], dp[1][q] + inputs[i][q - 1] - 1);
			}
			dp[1][q + 1] = max(dp[1][q + 1], dp[2][q] + 1);
			dp[2][q + 1] = dp[0][q] + inputs[i][q] - 1;
			if (inputs[i][q - 1] != 1) {
				dp[2][q + 1] = max(dp[2][q + 1], dp[1][q] + inputs[i][q] - 1);
			}
			dp[2][q + 1] = max(dp[2][q + 1], dp[2][q] + inputs[i][q] - 1);
			dp[3][q + 1] = dp[0][q] + inputs[i][q];
			if (inputs[i][q-1] != 1) {
				dp[3][q + 1] = max(dp[3][q + 1], dp[1][q] + inputs[i][q]);
			}
			dp[3][q + 1] = max(dp[3][q + 1], dp[2][q] + inputs[i][q]);
		}
		int hoge = inputs[i].size();
		ans += max({ dp[0][hoge],dp[1][hoge],dp[2][hoge],dp[3][hoge] });
		REP(q, inputs[i].size() + 1) {
			REP(j, 4) {
				dp[j][q] = 0;
			}
		}
	}
	cout << ans << endl;

}

Measured memory_bytes: 8568