#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <cstdio>
#include <cstring>
#include <iterator>
#include <bitset>
#include <unordered_set>
#include <unordered_map>
#include <fstream>
#include <iomanip>
#include <cassert>
#include <utility>
#include <memory>
#include <functional>
#include <deque>
#include <cctype>
#include <ctime>
#include <numeric>
#include <list>
#include <iomanip>

#if __cplusplus >= 201103L
#include <array>
#include <tuple>
#include <initializer_list>
#include <forward_list>

#define cauto const auto&
#else

#endif

using namespace std;


typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef vector<int> vint;
typedef vector<vector<int> > vvint;
typedef vector<long long> vll;
typedef vector<vector<long long> > vvll;

#define VV(T) vector<vector< T > >

template <class T>
void initvv(vector<vector<T> > &v, int a, int b, const T &t = T()){
    v.assign(a, vector<T>(b, t));
}

template <class F, class T>
void convert(const F &f, T &t){
    stringstream ss;
    ss << f;
    ss >> t;
}

#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i,n) _rep2((i),0,(n))
#define _rep2(i,a,b) for(int i=(a);i<(b);++i)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define fi first
#define se second
#define mkp make_pair
#define DEBUG
#ifdef DEBUG
#define dump(x)  cout << #x << " = " << (x) << endl;
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
#else
#define dump(x) 
#define debug(x) 
#endif

#define MOD 1000000007LL
#define EPS 1e-8
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
#define maxs(x,y) x=max(x,y)
#define mins(x,y) x=min(x,y)

int func(string s){
	vint v;
	rep(i,s.size()){
		if(s.substr(i,2)=="00"){
			assert(0);
		}
	}
	assert(s[0]!='0');
	assert(s[s.size()-1]!='0');
	// cout<<s<<endl;
	int l = 0;
	int ret = 0;
	rep(i,s.size()){
		if(s[i]=='0'){
			v.PB(i-1-l+1);
			assert(i-l);
			l=i+1;
		}
	}
	v.PB(s.size()-l);
	if(v.size()==1) return 0;
	int n = v.size();
	int m=4;
	vint dp(m);
	rep(i,1,n){
		vint next(m);
		rep(j,4) maxs(next[3], dp[j]);
		// 0  1  2    3
		// 0  1  x-1  x
		// maxs(next[], dp[0]);
		maxs(next[2], dp[1]+1);
		maxs(next[2], dp[2]+v[i-1]-1);
		maxs(next[2], dp[3]+v[i-1]);

		// maxs(next[1], dp[0]);
		maxs(next[1], dp[1]+v[i]-1);
		if(v[i-1]>1) maxs(next[1], dp[2]+v[i]-1);
		maxs(next[1], dp[3]+v[i]-1);

		// maxs(next[0], dp[0]);
		maxs(next[0], dp[1]+v[i]);
		if(v[i-1]>1) maxs(next[0], dp[2]+v[i]);
		maxs(next[0], dp[3]+v[i]);
		dp=next;
	}
	int ans = 0;
	rep(i,m) maxs(ans, dp[i]);
	// cout<<"ans "<<s<<" "<<ans<<endl;
	return ans;
}

int mainmain(int n,string s){
	// int n;
	// cin>>n;
	// string s;
	// cin>>s;
	s="00"+s+"00";
	int l = 0;
	int ans = 0;
	if(s.substr(0,2)=="00"){
		int p = 0;
		while(p<s.size()&&s[p]=='0') p++;
		s=s.substr(p);
		n=s.size();
	}
	// cout<<s<<endl;
	rep(i,n){
		// cout<<i<<endl;
		if(s[i]=='0'&&s[i+1]=='0'){
			// if(s[l]=='0'){
				// cout<<"!!!     "<<l<<" "<<s<<endl;
			// }
			ans += func(s.substr(l, (i-1)-l+1));
			while(i<n&&s[i]=='0'){
				i++;
			}
			l=i;
		}
	}
	// cout<<ans<<endl;
	return ans;
}


signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout<<fixed<<setprecision(20);
    int n;
    cin>>n;
    string s;
    cin>>s;
    cout<<mainmain(n,s)<<endl;
}

Measured memory_bytes: 1820