//#include <tourist>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> p;
const int INF = 1e9;
const ll LINF = ll(1e18) + 1;
const int MOD = 1000000007;
const int dx[4] = {0, 1, 0, -1}, dy[4] = {-1, 0, 1, 0};
const int Dx[8] = {0, 1, 1, 1, 0, -1, -1, -1}, Dy[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
#define yes cout << "Yes" << endl
#define YES cout << "YES" << endl
#define no cout << "No" << endl
#define NO cout << "NO" << endl
#define rep(i, n) for (int i = 0; i < n; i++)
#define ALL(v) v.begin(), v.end()
#define debug(v)          \
    cout << #v << ":";    \
    for (auto x : v)      \
    {                     \
        cout << x << ' '; \
    }                     \
    cout << endl;
template <class T>
bool chmax(T &a, const T &b)
{
    if (a < b)
    {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T &a, const T &b)
{
    if (b < a)
    {
        a = b;
        return 1;
    }
    return 0;
}
//cout<<fixed<<setprecision(15);有効数字15桁
//-std=c++14
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
int n;
string s;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> n>>s;
    ll ans=0;
    ll onecount=0;
    ll ninecount=0;
    string t="";
    rep(i,n)
    {
        if(s[i]=='3'||s[i]=='7'||s[i]=='5'){
            ans++;
        }
        else{
            t+=s[i];
        }
    }
    queue<int> nine;
    vector<bool> used(t.size(),false);
    for(int i=t.size()-1;i>=0;i--){
        if(t[i]=='9')nine.push(i);
        else if(!nine.empty()&&t[i]=='1'){
            used[nine.front()]=true;
            nine.pop();
            used[i]=true;
            ans++;
        }
    }
    string v="";
    rep(i,t.size()){
        if(!used[i])v+=t[i];
    }
    rep(i,v.size()){
        if(v[i]=='9')ninecount++;
        else{
            if(ninecount>=2){
                ninecount-=2;
                ans++;
            }
            else onecount++;
        }
    }
    /*
    cout<<t<<"\n";
    cout<<u<<"\n";
    cout<<v<<"\n";
    */
    cout << ans+onecount/2 << "\n";
}