#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cctype>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>
#include<complex>

using namespace std;

#define ll long long
#define ld long double
#define EPS 1e-9
#define INF 1e9
#define LINF (ll)INF*INF
#define MOD 1000000007
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,a,n) for(int i=a;i<(n);i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

#define int ll //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int,int> pii;
typedef pair<pii,int> ppi;
typedef pair<int,pii> pip;
typedef vector<pii> vp;
typedef vector<vi> vvi;

int gcd(int a, int b){if(b==0) return a;return gcd(b,a%b);}
int lcm(int a, int b){return a/gcd(a,b)*b;}


signed main(void) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    //11,19,991
    int ans = 0;
    set<int> a,b;//1,9
    rep(i,n){
        switch (s[i]) {
            case '1': a.insert(i);break;
            case '9': b.insert(i);break;
            case '3':;
            case '5':;
            case '7':ans++;break;
        }
    }
    //cout << ans << endl;
    //for(auto x: a)cout << " "<< x;cout << endl;
    //for(auto x: b)cout << " "<< x;cout << endl;
    int p = a.size(), q = b.size();
    for(auto x: a){
        auto itr = b.lower_bound(x);
        int num1 = *itr;
        //cout << x << endl;
        //for(auto y:b)cout << " " << y;cout << endl;
        if(itr != b.end()){
            ans++;//19
            p--,q--;
            b.erase(num1);
        }
    }
    //cout << ans << " " << p <<  " " << q << endl;
    //9…9911…1 , q,p
    int t = min(q/2,p);//991の個数
    ans += t;
    p -= t;
    ans += p/2;
    cout << ans << endl;
}