//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,avx512f")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <list>
#include <stack>
#include <queue>
#include <bitset>
#include <numeric>
#include <cassert>
#include <memory>
#include <random>
#include <functional>
#include <complex>
#include <immintrin.h>
#ifdef DEBUG
#include "./Lib/debug_VC.hpp"
#include "./Lib/Timer.hpp"
#include "./Lib/sample.hpp"
#else
#define dump(...)
#endif

/* macro */
#define FOR(i, b, e) for(ll i = (ll)(b); i < (ll)(e); ++i)
#define RFOR(i, b, e) for(ll i = (ll)(e-1); i >= (ll)(b); --i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define REPC(x,c) for(const auto& x:(c))
#define REPI2(it,b,e) for(auto it = (b); it != (e); ++it)
#define REPI(it,c) REPI2(it, (c).begin(), (c).end())
#define RREPI(it,c) REPI2(it, (c).rbegin(), (c).rend())
#define REPI_ERACE2(it, b, e) for(auto it = (b); it != (e);)
#define REPI_ERACE(it, c) REPI_ERACE2(it, (c).begin(), (c).end())
#define ALL(x) (x).begin(),(x).end()
#define cauto const auto&
/* macro func */
template<class T>
inline auto sort(T& t) { std::sort(ALL(t)); }
template<class T>
inline auto rsort(T& t) { std::sort((t).rbegin(), (t).rend()); }
template<class T>
inline auto unique(T& t) { (t).erase(unique((t).begin(), (t).end()), (t).end()); }
template<class T, class S>
inline auto chmax(T& t, const S& s) { if (s > t) { t = s; return true; } return false; }
template<class T, class S>
inline auto chmin(T& t, const S& s) { if (s < t) { t = s; return true; } return false; }
inline auto BR() { std::cout << "\n"; }

/* type define */
using ll = long long;
using PAIR = std::pair<ll, ll>;
using VS = std::vector<std::string>;
using VL = std::vector<long long>;
using VVL = std::vector<VL>;
using VVVL = std::vector<VVL>;
using VD = std::vector<double>;
template<class T>
using V = std::vector<T>;

/* using std */
using std::cout;
constexpr char endl = '\n';
using std::cin;
using std::pair;
using std::string;
using std::stack;
using std::queue;
using std::vector;
using std::list;
using std::map;
using std::unordered_map;
using std::multimap;
using std::unordered_multimap;
using std::set;
using std::unordered_set;
using std::unordered_multiset;
using std::multiset;
using std::bitset;
using std::priority_queue;

/* Initial processing  */
struct Preprocessing { Preprocessing() { std::cin.tie(0); std::ios::sync_with_stdio(0); }; }_Preprocessing;

/* Remove the source of the bug */
signed pow(signed, signed) { assert(false); return -1; }

/* define hash */
namespace std {
template <>	class hash<std::pair<ll, ll>> { public:	size_t operator()(const std::pair<ll, ll>& x) const { return hash<ll>()(1000000000 * x.first + x.second); } };
}

/* input */
template<class T> std::istream& operator >> (std::istream& is, vector<T>& vec) { for (T& x : vec) is >> x; return is; }

/* constant value */
//constexpr ll MOD = 1000000007;
constexpr ll MOD = 998244353;

//=============================================================================================;


class CumulativeSum {
	const ll size;
	std::vector<ll> sumList;
public:

	CumulativeSum() = delete;
	CumulativeSum(const std::vector<ll>& v) :size(v.size()), sumList(size + 1) {
		for (ll i = 0; i < size; ++i) {
			sumList[i + 1] = sumList[i] + v[i];
		}
	}
	ll get(ll n) {
		return sumList[n + 1];
	}
	ll get(ll l, ll r) {
		if (r < l) { return 0LL; }
		l = std::max(l, 0LL); r = std::min(r, size - 1);
		return sumList[r + 1] - sumList[l];
	}
};


signed main() {

	auto solve = [](ll n, const list<ll>& lst) {
		if (n == 1) { return 0LL; }
		VL v; v.reserve(n);
		REPC(x, lst) { v.emplace_back(x); }
		VL dp1(n - 1);// 左
		VL dp2(n - 1);// 無
		VL dp3(n - 1);// 右
		VL dp4(n - 1);// 右1残し
		dp1[0] = v[0];
		dp2[0] = 0;
		dp3[0] = v[1];
		if (v[1] > 1) { dp4[0] = v[1] - 1; }
		FOR(i, 1, n - 1) {
			// 左
			if (v[i] > 1) { chmax(dp1[i], dp1[i - 1] + v[i] - 1); }
			chmax(dp1[i], dp2[i - 1] + v[i]);

			chmax(dp1[i], dp4[i - 1] + 1);
			// 無
			chmax(dp2[i], dp1[i - 1]);
			chmax(dp2[i], dp2[i - 1]);
			chmax(dp2[i], dp3[i - 1]);
			chmax(dp2[i], dp4[i - 1]);
			// 右
			if (v[i] > 1) { chmax(dp3[i], dp1[i - 1] + v[i + 1]); }
			chmax(dp3[i], dp2[i - 1] + v[i + 1]);

			chmax(dp3[i], dp4[i - 1] + v[i + 1]);
			// 右1残し
			if (v[i] > 1 && v[i + 1] > 1) { chmax(dp4[i], dp1[i - 1] + v[i + 1] - 1); }
			if (v[i + 1] > 1) { chmax(dp4[i], dp2[i - 1] + v[i + 1] - 1); }

			if (v[i + 1] > 1) { chmax(dp4[i], dp4[i - 1] + v[i + 1] - 1); }

		}
		//dump(lst);
		//dump(dp1, dp2, dp3, dp4);
		ll ans = 0;
		chmax(ans, dp1[n - 2]);
		chmax(ans, dp2[n - 2]);
		chmax(ans, dp3[n - 2]);
		chmax(ans, dp4[n - 2]);
		return ans;
	};


	//		1 2 1 1 4
		//	5 3 3
			//2 2 1 3
			//1 3 1 1
		//1 3 1 2 1 
		//1 1 2 1 4 1 
		//1 1 3 1 2
			//1 1
			//1 1 1 1 2
	/**
	auto l = list<ll>{
		1,3,2,1,2

	};
	ll n = l.size();
	string s;
	s += '0';
	s += '0';
	REPC(x, l) { REP(_, x) { s += '1'; }s += '0'; }
	s += '0';
	REPC(x, l) { REP(_, x) { s += '1'; }s += '0'; }
	cout << s.size() << endl;
	cout << s << endl;
	auto tre = solve(n, l);
	dump(tre);
	BR(); BR();
	/*/
	ll n;
	cin >> n;
	string s;
	cin >> s;
	/**/
	ll ans = 0;
	list<ll> lst;
	bool zero = false;
	ll cnt = 0;
	REPC(c, s) {
		if (c == '0') {
			if (zero) {
				if (!lst.empty()) { ans += solve(lst.size(), lst); }
				zero = false;
				lst = list<ll>();
			} else {
				if (cnt > 0) { lst.emplace_back(cnt); }
				zero = true;
			}
			cnt = 0;
		} else {
			if (zero) {
				cnt = 1;
			} else {
				++cnt;
			}
			zero = false;
		}
	}
	if (cnt > 0) { lst.emplace_back(cnt); }
	if (!lst.empty()) { ans += solve(lst.size(), lst); }

	cout << ans << endl;
}

Measured memory_bytes: 1172