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

#define dump(...) do{print_vars(cout<<"# "<<#__VA_ARGS__<<'=',__VA_ARGS__);cout<<endl;}while(0)
#define repi(i,a,b) for(int i=int(a);i<int(b);i++)
#define peri(i,a,b) for(int i=int(b);i-->int(a);)
#define rep(i,n) repi(i,0,n)
#define per(i,n) peri(i,0,n)
#define all(c) begin(c),end(c)
#define mp make_pair
#define mt make_tuple

using uint=unsigned;
using ll=long long;
using ull=unsigned long long;
using vi=vector<int>;
using vvi=vector<vi>;
using vl=vector<ll>;
using vvl=vector<vl>;
using vd=vector<double>;
using vvd=vector<vd>;
using vs=vector<string>;

void print_vars(ostream&){}
template<typename Car,typename... Cdr>
void print_vars(ostream& os,const Car& car,const Cdr&... cdr){
	print_vars(os<<car<<(sizeof...(cdr)?",":""),cdr...);
}

template<typename T1,typename T2>
ostream& operator<<(ostream& os,const pair<T1,T2>& p){
	return os<<'('<<p.first<<','<<p.second<<')';
}

template<int I,typename Tuple>
void print_tuple(ostream&,const Tuple&){}
template<int I,typename Car,typename... Cdr,typename Tuple>
void print_tuple(ostream& os,const Tuple& t){
	os<<get<I>(t)<<(sizeof...(Cdr)?",":"");
	print_tuple<I+1,Cdr...>(os,t);
}
template<typename... Args>
ostream& operator<<(ostream& os,const tuple<Args...>& t){
	print_tuple<0,Args...>(os<<'(',t);
	return os<<')';
}

template<typename Ch,typename Tr,typename C>
basic_ostream<Ch,Tr>& operator<<(basic_ostream<Ch,Tr>& os,const C& c){
	os<<'[';
	for(auto i=begin(c);i!=end(c);++i)
		os<<(i==begin(c)?"":" ")<<*i;
	return os<<']';
}

constexpr int INF=1e9;
constexpr int MOD=1e9+7;
constexpr double EPS=1e-9;

int NextPow2(int x)
{
	x--;
	for(int i=1;i<32;i*=2) x|=x>>i;
	return x+1;
}
const int I=0;
int F(int a,int b){return max(a,b);}

struct SegmentTree{
	int size;
	vi data;
	SegmentTree(int n):size(NextPow2(n)),data(size*2,I){}
	SegmentTree(const vi& a):size(NextPow2(a.size())),data(size*2,I){
		copy(all(a),begin(data)+size);
		peri(i,1,size) data[i]=F(data[i*2],data[i*2+1]);
	}
	int Get(int i){
		return data[size+i];
	}
	void Update(int i,int x){
		data[size+i]=x;
		for(i+=size;i/=2;) data[i]=F(data[i*2],data[i*2+1]);
	}
	int Query(int a,int b,int i,int l,int r){
		if(b<=l || r<=a) return I;
		if(a<=l && r<=b) return data[i];
		return F(Query(a,b,i*2,l,(l+r)/2),Query(a,b,i*2+1,(l+r)/2,r));
	}
	int Query(int a,int b){
		return Query(a,b,1,0,size);
	}
};

int calc(const string& s)
{
	vector<tuple<int,int>> ranges;
	for(int i=0;(i=s.find("101",i))!=-1;i++){
		int l=i,r=i+3;
		while(l-1>=0&&s[l-1]=='1') l--;
		while(r<s.size()&&s[r]=='1') r++;
		// 1011..11
		ranges.emplace_back(i,r);
		if(i+3<r) ranges.emplace_back(i,r-1);
		// 11..1101
		ranges.emplace_back(l,i+3);
		if(l<i) ranges.emplace_back(l+1,i+3);
	}
	sort(all(ranges),[](auto a,auto b){
		return get<1>(a)<get<1>(b);
	});

	SegmentTree st(s.size());
	for(auto range:ranges){
		int l,r; tie(l,r)=range;
		int cand=st.Query(0,l)+(r-l-2);
		if(st.Get(r-1)<cand)
			st.Update(r-1,cand);
	}
	return st.Query(0,s.size());
}

int solve(const string& s)
{
	int res=0;
	for(int i=0,j=0;i!=-1;i=j){
		if((i=s.find('1',i))==-1)
			break;
		j=s.find("00",i);
		string t=j==-1?s.substr(i):s.substr(i,j-i);
		res+=calc(t);
	}
	return res;
}

int main()
{
	#ifndef _GLIBCXX_DEBUG
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	constexpr char endl='\n';
	#endif

	for(int n;cin>>n&&n;){
		string s; cin>>s;
		cout<<solve(s)<<endl;
	}
}


Measured memory_bytes: 5404