#include <bits/stdc++.h>
                            #include<iostream>
                            #include<cstdio>
                            #include<vector>
                            #include<queue>
                            #include<map>
                            #include<cstring>
                            #include<string>
                            #include <math.h>
                            #include<algorithm>
                        //    #include <boost/multiprecision/cpp_int.hpp>
                            #include<functional>
                    #define int long long
                            #define inf  1000000007
                            #define pa pair<int,int>
                            #define ll long long
                            #define pal pair<double,pa>
                            #define ppa pair<int,int>
                            #define ppap pair<int,pa>
                            #define ssa pair<string,int>
                            #define  mp make_pair
                            #define  pb push_back
                            #define EPS (1e-10)
                            #define equals(a,b) (fabs((a)-(b))<EPS)
                     
                            using namespace std;
                  
                            class Point{
                            	public:
                            	double x,y;
                            	Point(double x=0,double y=0):x(x),y(y) {}
                            	Point operator + (Point p) {return Point(x+p.x,y+p.y);}
                            	Point operator - (Point p) {return Point(x-p.x,y-p.y);}
                            	Point operator * (double a) {return Point(x*a,y*a);}
                            	Point operator / (double a) {return Point(x/a,y/a);}
                            	double absv() {return sqrt(norm());}
                            	double norm() {return x*x+y*y;}
                            	bool operator < (const Point &p) const{
                            		return x != p.x ? x<p.x: y<p.y;
                            	}
                            	bool operator == (const Point &p) const{
                            		return fabs(x-p.x)<EPS && fabs(y-p.y)<EPS;
                            	}
                            };
                            typedef Point Vector;
                     
                            struct Segment{
                            Point p1,p2;
                            };
                             double dot(Vector a,Vector b){
                            	return a.x*b.x+a.y*b.y;
                            }
                            double cross(Vector a,Vector b){
                            	return a.x*b.y-a.y*b.x;
                            }
                        
bool parareru(Point a,Point b,Point c,Point d){
//	if(abs(cross(a-b,d-c))<EPS)cout<<"dd "<<cross(a-b,d-c)<<endl;
	return abs(cross(a-b,d-c))<EPS;
}
double distance_ls_p(Point a, Point b, Point c) {
  if ( dot(b-a, c-a) < EPS ) return (c-a).absv();
  if ( dot(a-b, c-b) < EPS ) return (c-b).absv();
  return abs(cross(b-a, c-a)) / (b-a).absv();
}
bool is_intersected_ls(Segment a,Segment b) {
	if(a.p1==b.p1||a.p2==b.p1||a.p1==b.p2||a.p2==b.p2) return false;
	if(parareru((a.p2),(a.p1),(a.p1),(b.p2))&&parareru((a.p2),(a.p1),(a.p1),(b.p1))){
//		cout<<"sss"<<endl;
		if(dot(a.p1-b.p1,a.p1-b.p2)<EPS) return true;
		if(dot(a.p2-b.p1,a.p2-b.p2)<EPS) return true;
		if(dot(a.p1-b.p1,a.p2-b.p1)<EPS) return true;
		if(dot(a.p1-b.p2,a.p2-b.p2)<EPS) return true;
		return false;
	}
  else return ( cross(a.p2-a.p1, b.p1-a.p1) * cross(a.p2-a.p1, b.p2-a.p1) < EPS ) && ( cross(b.p2-b.p1, a.p1-b.p1) * cross(b.p2-b.p1, a.p2-b.p1) < EPS );
}
 
double segment_dis(Segment a,Segment b){
	if(is_intersected_ls(a,b))return 0;
	double r=distance_ls_p(a.p1, a.p2, b.p1);
	r=min(r,distance_ls_p(a.p1, a.p2, b.p2));
	r=min(r,distance_ls_p(b.p1, b.p2, a.p2));
	r=min(r,distance_ls_p(b.p1, b.p2, a.p1));
	return r;
}
Point intersection_ls(Segment a, Segment b) {
  Point ba = b.p2-b.p1;
  double d1 = abs(cross(ba, a.p1-b.p1));
  double d2 = abs(cross(ba, a.p2-b.p1));
  double t = d1 / (d1 + d2);
 
  return a.p1 + (a.p2-a.p1) * t;
}
 
                string itos( int i ) {
                ostringstream s ;
                s << i ;
                return s.str() ;
                }
                 
                int gcd(int v,int b){
                	if(v>b) return gcd(b,v);
                	if(v==b) return b;
                	if(b%v==0) return v;
                	return gcd(v,b%v);
                }
 
                double distans(double x1,double y1,double x2,double y2){
                	double rr=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
                	return sqrt(rr);
                	
                }
                /*
int pr[100010];
//int inv[100010];
*/ 
int beki(int wa,int rr,int warukazu){
	if(rr==0) return 1ll;
	if(rr==1) return wa%warukazu;
	if(rr%2==1) return (beki(wa,rr-1,warukazu)*wa)%warukazu;
	int zx=beki(wa,rr/2,warukazu);
	return (zx*zx)%warukazu;
}
  /*
void gya(){
	pr[0]=1;
	for(int i=1;i<100010;i++){
		pr[i]=(pr[i-1]*i)%inf;
	}
	for(int i=0;i<100010;i++) inv[i]=beki(pr[i],inf-2);
	
}
 
*/
 
                         //----------------kokomade tenpure------------
//vector<double> ans(100000000),ans2(100000000);

int a,c;
int n,k,m;
map<int,int> d;
string s,t;

int dp[500002][5]={0};
vector<int> ko;
vector<vector<int>> ve;
signed main(){
	
	
	
cin>>n;
	cin>>s;
	
	s+="0001";
	n+=4;
	int r=0;
	
	while(s[r]=='0') r++;
	int ka=0;
int	mae=1;

	while(1){
	
	//	if(r==0)cout<<s[0]<<" "<<mae<<endl;
		if((s[r]-'0')==mae){
		//	if(r==0)cout<<"d"<<endl;
			ka++;
			mae=s[r]-'0';
		}
		else{
	//		if(r==0)cout<<"d2"<<endl;
			ko.pb(ka);
	//		cout<<r<<" "<<ka<<endl;
			ka=1;
			mae=s[r]-'0';
		}
		r++;
		if(r>=n) break;
	}
	
//	for(int i=0;i<ko.size();i++) cout<<ko[i]<<" ";
//	cout<<endl;
	
	vector<int> vec;
	for(int i=0;i<ko.size();i+=2){
		vec.pb(ko[i]);
		if(ko[i+1]==1) continue;
		else{
			ve.pb(vec);
			vec.clear();
		}
	}
//	cout<<ve.size()<<endl;
	
	int ans=0;
	
	for(int i=0;i<ve.size();i++){
		if(ve[i].size()==1) continue;
		if(ve[i].size()==2) {
			ans+=max(ve[i][0],ve[i][1]);
			continue;
		}
		vector<int> w=ve[i];
	//	for(int i=0;i<w.size();i++) cout<<w[i]<<" ";
	//	cout<<endl;
		int l=w.size();
		
		dp[0][0]=0;
		dp[0][1]=0;
		dp[0][2]=0;
		dp[0][3]=0;
		for(int i=1;i<=l-1;i++){
		//	cout<<i<<" "<<w[i]<<endl;
			dp[i][1]=0;
			dp[i][1]=max(dp[i-1][2]+w[i],dp[i][1]);
			if(w[i-1]>1)dp[i][1]=max(dp[i-1][3]+w[i],dp[i][1]);
			dp[i][1]=max(dp[i-1][4]+w[i],dp[i][1]);
		
			dp[i][2]=0;
			dp[i][2]=max(dp[i-1][2]+w[i]-1,dp[i][2]);
			if(w[i-1]>1)dp[i][2]=max(dp[i-1][3]+w[i]-1,dp[i][2]);
			dp[i][2]=max(dp[i-1][4]+w[i]-1,dp[i][2]);
			
			dp[i][3]=0;
			dp[i][3]=max(dp[i-1][4]+w[i-1],dp[i][3]);
			dp[i][3]=max(dp[i-1][3]+w[i-1]-1,dp[i][3]);
			dp[i][3]=max(dp[i-1][2]+1,dp[i][3]);
			
			dp[i][4]=0;
			dp[i][4]=max(dp[i-1][1],dp[i][4]);
			dp[i][4]=max(dp[i-1][2],dp[i][4]);
			dp[i][4]=max(dp[i-1][3],dp[i][4]);
			dp[i][4]=max(dp[i-1][4],dp[i][4]);
			//for(int j=1;j<=4;j++)	cout<<i<<" "<<j<<" "<<dp[i][j]<<endl;
		}
		
		int u=0;
		for(int i=1;i<=4;i++)u=max(u,dp[l-1][i]);
		ans+=u;
	//	cout<<u<<endl;
		for(int i=0;i<=l;i++)for(int j=0;j<5;j++)dp[i][j]=0;
		
	}
	
	cout<<ans<<endl;
	return 0;
}

Measured memory_bytes: 7800