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

#define int long long

#define rep(i,n) for(int i=0;i<(n);i++)
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}

template<class A,class B>
ostream& operator<<(ostream& ost,const pair<A,B>&p){
	ost<<"{"<<p.first<<","<<p.second<<"}";
	return ost;
}

template<class T>
ostream& operator<<(ostream& ost,const vector<T>&v){
	ost<<"{";
	for(int i=0;i<v.size();i++){
		if(i)ost<<",";
		ost<<v[i];
	}
	ost<<"}";
	return ost;
}



signed main(){
	int N;
	string S;
	cin>>N>>S;

	int ans=0;
	vint v;

	int z=0;
	int n=0;

	rep(i,N){
		if(S[i]=='1'){
			if(n>=2){
				n-=2;
				ans++;
			}
			else z++;
		}
		else if(S[i]=='9'){
			if(z){
				z--;
				ans++;
			}
			else n++;
		}
		else ans++;
	}
	ans+=z/2;
	cout<<ans<<endl;

	return 0;
}