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

// {{{
// clang-format off
#define forr(x,arr) for(auto&& x:arr)
#define _overload3(_1,_2,_3,name,...) name
#define _rep2(i,n) _rep3(i,0,n)
#define _rep3(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,_rep3,_rep2,)(__VA_ARGS__)
#define _rrep2(i,n) _rrep3(i,0,n)
#define _rrep3(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define rrep(...) _overload3(__VA_ARGS__,_rrep3,_rrep2,)(__VA_ARGS__)
#define all(x) (x).begin(),(x).end()
#define bit(n) (1LL<<(n))
#define sz(x) ((int)(x).size())
#define TEN(n) ((ll)(1e##n))
#define fst first
#define snd second

string DBG_DLM(int &i){return(i++==0?"":", ");}
#define DBG_B(exp){int i=0;os<<"{";{exp;}os<<"}";return os;}
template<class T>ostream&operator<<(ostream&os,vector<T>v);
template<class T>ostream&operator<<(ostream&os,set<T>v);
template<class T>ostream&operator<<(ostream&os,queue<T>q);
template<class T>ostream&operator<<(ostream&os,priority_queue<T>q);
template<class T,class K>ostream&operator<<(ostream&os,pair<T,K>p);
template<class T,class K>ostream&operator<<(ostream&os,map<T,K>mp);
template<class T,class K>ostream&operator<<(ostream&os,unordered_map<T,K>mp);
template<int I,class TPL>void DBG(ostream&os,TPL t){}
template<int I,class TPL,class H,class...Ts>void DBG(ostream&os,TPL t){os<<(I==0?"":", ")<<get<I>(t);DBG<I+1,TPL,Ts...>(os,t);}
template<class T,class K>void DBG(ostream&os,pair<T,K>p,string delim){os<<"("<<p.first<<delim<<p.second<<")";}
template<class...Ts>ostream&operator<<(ostream&os,tuple<Ts...>t){os<<"(";DBG<0,tuple<Ts...>,Ts...>(os,t);os<<")";return os;}
template<class T,class K>ostream&operator<<(ostream&os,pair<T,K>p){DBG(os,p,", ");return os;}
template<class T>ostream&operator<<(ostream&os,vector<T>v){DBG_B(forr(t,v){os<<DBG_DLM(i)<<t;});}
template<class T>ostream&operator<<(ostream&os,set<T>s){DBG_B(forr(t,s){os<<DBG_DLM(i)<<t;});}
template<class T>ostream&operator<<(ostream&os,queue<T>q){DBG_B(for(;q.size();q.pop()){os<<DBG_DLM(i)<<q.front();});}
template<class T>ostream&operator<<(ostream&os,priority_queue<T>q){DBG_B(for(;q.size();q.pop()){os<<DBG_DLM(i)<<q.top();});}
template<class T,class K>ostream&operator<<(ostream&os,map<T,K>m){DBG_B(forr(p,m){os<<DBG_DLM(i);DBG(os,p,"->");});}
template<class T,class K>ostream&operator<<(ostream&os,unordered_map<T,K>m){DBG_B(forr(p,m){os<<DBG_DLM(i);DBG(os,p,"->");});}
#define DBG_OVERLOAD(_1,_2,_3,_4,_5,_6,macro_name,...)macro_name
#define DBG_LINE(){char s[99];sprintf(s,"line:%3d | ",__LINE__);cerr<<s;}
#define DBG_OUTPUT(v){cerr<<(#v)<<"="<<(v);}
#define DBG1(v,...){DBG_OUTPUT(v);}
#define DBG2(v,...){DBG_OUTPUT(v);cerr<<", ";DBG1(__VA_ARGS__);}
#define DBG3(v,...){DBG_OUTPUT(v);cerr<<", ";DBG2(__VA_ARGS__);}
#define DBG4(v,...){DBG_OUTPUT(v);cerr<<", ";DBG3(__VA_ARGS__);}
#define DBG5(v,...){DBG_OUTPUT(v);cerr<<", ";DBG4(__VA_ARGS__);}
#define DBG6(v,...){DBG_OUTPUT(v);cerr<<", ";DBG5(__VA_ARGS__);}
#define DEBUG0(){DBG_LINE();cerr<<endl;}
#ifdef LOCAL
#define out(...){DBG_LINE();DBG_OVERLOAD(__VA_ARGS__,DBG6,DBG5,DBG4,DBG3,DBG2,DBG1)(__VA_ARGS__);cerr<<endl;}
#else
#define out(...)
#endif

using ll=long long;
using pii=pair<int,int>;using pll=pair<ll,ll>;using pil=pair<int,ll>;using pli=pair<ll,int>;
using vs=vector<string>;using vvs=vector<vs>;using vvvs=vector<vvs>;
using vb=vector<bool>;using vvb=vector<vb>;using vvvb=vector<vvb>;
using vi=vector<int>;using vvi=vector<vi>;using vvvi=vector<vvi>;
using vl=vector<ll>;using vvl=vector<vl>;using vvvl=vector<vvl>;
using vd=vector<double>;using vvd=vector<vd>;using vvvd=vector<vvd>;
using vpii=vector<pii>;using vvpii=vector<vpii>;using vvvpii=vector<vvpii>;
template<class A,class B>bool amax(A&a,const B&b){return b>a?a=b,1:0;}
template<class A,class B>bool amin(A&a,const B&b){return b<a?a=b,1:0;}
ll ri(){ll l;cin>>l;return l;} string rs(){string s;cin>>s;return s;}
// clang-format on
// }}}

ll solve() {
	ri();
	auto s = rs();
	int n = sz(s);

	if (n < 3) return 0;

	int ans = 0;

	vi B;
	B.emplace_back(0);
	forr(c, s) {
		if (c == '0') {
			B.emplace_back(0);
		}
		else {
			B.back()++;
		}
	}

	int m = sz(B);
	out(B, m);
	B.emplace_back(0);

	vvi dp(m + 1, vi(3, -1));
	dp[0][0] = 0;

	// 0: 貸し借りなし
	// 1: iは左に貸りた
	// 2: iは左に貸した

	rep(i, m) {
		int b = B[i];
		int c = B[i+1];
		out(i, b, c, dp[i]);

		// 貸し借りなし
		if (dp[i][0] >= 0) {
			amax(dp[i+1][0], dp[i][0]);
			if (b>0) amax(dp[i+1][1], dp[i][0]);  // 右に貸す
			if (c>0) amax(dp[i+1][2], dp[i][0] + b);  // 右に借りる
		}

		// 左に借りた
		if (dp[i][1] >= 0) {
			amax(dp[i+1][0], dp[i][1] + b);  // 全部使う
			if (b>0) amax(dp[i+1][1], dp[i][1] + b - 1);  // 右に借す
		}

		// 左に借した
		if (dp[i][2] >= 0) {
			amax(dp[i+1][0], dp[i][2]);
			if (b>1) amax(dp[i+1][1], dp[i][2]);  // 右に借す
			if (c>0) amax(dp[i+1][2], dp[i][2] + b - 1);  // 右に借りる
		}
	}
	out(dp);

	ans = *max_element(all(dp[m]));

	return ans;
}

void Main() {
	cout << solve() << endl;
}

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  Main();
  return 0;
}


Measured memory_bytes: 30100