#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

// 3,5,7
// 11,19,
// 991

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    int n;cin>>n;
    string s;cin>>s;

    string t="";
    int ans=0;
    for(auto c:s){
        if(c=='3' or c=='5' or c=='7') ans++;
        else t.push_back(c);
    }
    if(t.empty()){
        cout<<ans<<endl;
        return 0;
    }

    int m=(int)t.size();
    int a=0,b=0;
    rep(i,m){
        if(t[i]=='1'){
            if(b>=2){
                b-=2;ans++;
            }else if(a>0){
                a--;ans++;
            }else{
                a++;
            }
        }else{
            if(a>0){
                a--;ans++;
            }else{
                b++;
            }
        }
    }
    cout<<ans<<endl;
    return 0;
}