#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("unroll-loops")
//#pragma warning(disable : 4996)




#ifdef _MSC_VER
#include <intrin.h>

#define __builtin_popcount __popcnt
#define __builtin_popcountll __popcnt64
#endif

#include <limits.h>
#include <math.h>
#include <time.h>

#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>



using namespace std;
//using namespace atcoder;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n - 1; i >= 0; --i)
#define FOR(i, m, n) for (int i = m; i < n; ++i)
#define FORR(i, m, n) for (int i = m - 1; i >= n; --i)
#define SORT(v, n) sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define REVERSE(v, n) reverse(v, v + n);
#define VREVERSE(v) reverse(v.begin(), v.end())
#define ll long long
#define print(x) cout << (x) << endl
#define pe(x) cout << (x) << " "
#define DEBUG(x) cout << #x << ": " << x << endl
#define lb(v, n) lower_bound(v.begin(), v.end(), (n))
#define ub(v, n) upper_bound(v.begin(), v.end(), (n))
#define int long long
//#define double long double
#define all(x) (x).begin(), (x).end()
#define print_space(v) REP(i, v.size()) cout << v[i] << " \n"[i==(int)v.size()-1]
template <typename T1, typename T2> inline void chmin(T1& a, T2 b) { if (a > b) a = b; }
template <typename T1, typename T2> inline void chmax(T1& a, T2 b) { if (a < b) a = b; }
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
std::random_device rd;
std::mt19937 mt(rd());
constexpr ll MOD = 1e9 + 7;
constexpr int MAX = 500050;
const double pi = acos(-1);
constexpr double EPS = 1e-8;
constexpr ll LINF = 1e18 + 1;
constexpr int INF = 1e9 + 1;
void Yes(bool cond) { cout << (cond ? "Yes" : "No") << '\n'; }
void YES(bool cond) { cout << (cond ? "YES" : "NO") << '\n'; }


int dp[500050][4];
void solve() {
	int N; cin >> N;
	string S;
	cin >> S;
	S += "00";
	N += 2;
	int cnt = 0;
	vector<vector<int>>vv;
	vector<int>v;
	REP(i, N) {
		if (S[i] == '0') {
			if (cnt == 0) {
				vv.push_back(v);
				v.clear();
			}
			else {
				v.push_back(cnt);
				cnt = 0;
			}
		}
		else {
			cnt++;
		}
	}
	ll ans = 0;
	for (auto A : vv) {
		//VSORT(A);
		//print_space(A);
		if (A.size() <= 1)continue;
		//FOR(i, 1, (int)A.size() - 1)A[i]--;
		//VSORT(A);
		int res = 0;
		//FOR(i, 1, A.size()) {
		//	res += A[i];
		//}
		//DEBUG(res);
		//if (res > 0)ans += res;
		int M = A.size();
		REP(i, M + 1)REP(j, 4)dp[i][j] = -INF;
		dp[1][0] = 0;//全部残す
		dp[1][1] = A[0];//A[1]-1個残す
		dp[1][2] = A[1] - 1;//1個残す
		dp[1][3] = A[1];//残さない
		FOR(i, 1, M - 1) {
			REP(j, 4) {
				if (j == 0) {
					chmax(dp[i + 1][0], dp[i][j]);
					chmax(dp[i + 1][1], dp[i][j]+A[i]);
					chmax(dp[i + 1][2], dp[i][j] + A[i + 1] - 1);
					chmax(dp[i + 1][3], dp[i][j]+A[i + 1]);
				}
				else if (j == 1) {
					chmax(dp[i + 1][0], dp[i][j]);
					chmax(dp[i + 1][1], dp[i][j] + A[i] - 1);
					if (A[i] > 1)chmax(dp[i + 1][2], dp[i][j] + A[i + 1] - 1);
					if (A[i] > 1)chmax(dp[i + 1][3], dp[i][j] + A[i + 1]);
				}
				else if (j == 2) {
					chmax(dp[i + 1][0], dp[i][j]);
					chmax(dp[i + 1][1], dp[i][j] + 1);
					chmax(dp[i + 1][2], dp[i][j] + A[i + 1] - 1);
					chmax(dp[i + 1][3], dp[i][j] + A[i + 1]);
				}
				else if (j == 3) {
					chmax(dp[i + 1][0], dp[i][j]);
				}
			}
		}
		REP(i, M)REP(j,4){
			//cerr << i << " " << j << " " << dp[i][j] << endl;
		}
		REP(i, M)REP(j, 4)chmax(res, dp[i][j]);
		//print(res);
		//cout << endl;
		ans += res;
	}
	print(ans);
}



signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	//int q;
	//cin >> q;
	//while (q--)
	solve();

}



Measured memory_bytes: 17784