#include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <algorithm>
#include <numeric>
#include <sstream>
#include <iostream>
#include <iomanip>
#define _USE_MATH_DEFINES
#include <cmath>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cfloat>
#include <cmath>
#include <ctime>
#include <cstdio>
#include <cassert>
using namespace std;

#define EPS 1e-12
#define ull unsigned long long
#define ll long long
#define VI vector<ll>
#define PII pair<ll, ll> 
#define VVI vector<vector<ll> >
#define REP(i,n) for(int i=0,_n=(n);(i)<(int)_n;++i)
#define RANGE(i,a,b) for(int i=(int)a,_b=(int)(b);(i)<_b;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define ALLR(c) (c).rbegin(), (c).rend()
#define PB push_back
#define MP(a, b) make_pair(a, b)
#define POPCOUNT __builtin_popcount
#define POPCOUNTLL __builtin_popcountll
#define CLEAR(table, v) memset(table, v, sizeof(table));
#define PRINT1(table, D0) REP(d0, D0) cout<<table[d0]<<" "; cout<<"\n";
#define PRINT2(table, D0, D1) REP(d0, D0) { REP(d1, D1) cout<<table[d0][d1]<<" "; cout<<"\n"; }
#define PRINT3(table, D0, D1, D2) REP(d0, D0) { REP(d1, D1) { REP(d2, D2) cout<<table[d0][d1][d2]<<" "; cout<<"\n"; } cout<<"\n"; }
#define UNIFORM_DOUBLE(a, b) (((b-a)*(double)rand()/RAND_MAX)+a) // [a, b) 
#define UNIFORM_LL(a, b) (ll)UNIFORM_DOUBLE(a, b) // [a, b) 
#define IN(v, lo, hi) ((lo)<=(v) && (v)<(hi))
#define DD(v) cout<<#v<<": "<<v<<endl
template <typename T0, typename T1> std::ostream& operator<<(std::ostream& os, const pair<T0, T1>& v) { os << v.first << ": " << v.second << " "; return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const vector<T>& v) { for( int i = 0; i < (int)v.size(); i++ ) { os << v[i] << " "; } return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const vector<vector<T> >& v) { for( int i = 0; i < (int)v.size(); i++ ) { os << v[i] << endl; } return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const set<T>& v) { vector<T> tmp(v.begin(), v.end()); os << tmp; return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const deque<T>& v) { vector<T> tmp(v.begin(), v.end()); os << tmp; return os; }
template <typename T0, typename T1> std::ostream& operator<<(std::ostream& os, const map<T0, T1>& v) { for( typename map<T0, T1>::const_iterator p = v.begin(); p!=v.end(); p++ ){os << p->first << ": " << p->second << " ";} return os; }

#define MOD 1000000007LL
#define INF (1LL<<60)

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	ll N;
	string s;
	while(cin>>N>>s) {
//		DD(s);
		VI dp(N);
		VI sr(N), sl(N);
		{
			ll c = 0;
			REP(i, N) {
				if(s[i]=='0') c=0;
				if(s[i]=='1') c++;
				sl[i]=c;
			}
		}
		{
			ll c = 0;
			REP(ii, N) {
				ll i = N-1-ii;
				if(s[i]=='0') c=0;
				if(s[i]=='1') c++;
				sr[i]=c;
			}
		}
		map<ll, vector<pair<ll, pair<ll, ll>>>> h;
		RANGE(i, 1, N-1) if(s[i]=='0' && s[i-1]=='1' && s[i+1]=='1') {
			VI ss, es;
			RANGE(s, i-1-sl[i-1]+1, i-1+1) {
				ll cost = max(1, i-1-s+1);
				ll e = i+1;
				h[e].PB(MP(s, MP(e, cost)));
			}
			RANGE(e, i+1, i+1+sr[i+1]-1+1) {
				ll cost = max(1, e-(i+1)+1);
				ll s = i-1;
				h[e].PB(MP(s, MP(e, cost)));
			}
		}
//		DD(h);
		REP(i, N) {
			dp[i] = i-1>=0 ? dp[i-1] : 0;
			if(!h.count(i)) continue;

			auto& rs = h[i];
			for(auto& v : rs) {
				// k ... j ... i
				// 1 1 1 0 1 1 1
				ll s = v.first;
				ll e = v.second.first;
				ll cost = v.second.second;
				dp[i] = max(dp[i], (s-1>=0 ? dp[s-1] : 0) + cost);
			}
		}
//		REP(i, N) {
//			dp[i] = i-1>=0 ? dp[i-1] : 0;
//			if(s[i]=='1') {
//				for(int j=i-1;j>=0;j--) {
//					if(s[j]=='0') {
//						for(int k=j-1;k>=0;k--) {
//							if(s[k]!='1') break;
//							// k ... j ... i
//							// 1 1 1 0 1 1 1
//							dp[i] = max(dp[i], (k-1>=0 ? dp[k-1] : 0) + max(j-k, i-j));
//						}
//						break;
//					}
//				}
//			}
//		}
//		DD(dp);
		cout<<dp[N-1]<<endl;
//		break;
	}
	
	return 0;
}


Measured memory_bytes: 82204