#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i,n) for(int (i)=((n)-1);(i)>=0;(i)--)
#define itn int
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
const long long INF = 1LL << 60;
const int MOD = 1000000007;
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
signed main(void){
    int n; cin>>n;
    string s; cin>>s;
    string t;
    int ans = 0;
    int one = -1, nine = -1;
    rep(i,n){
        if(s[i]=='3'|| s[i] == '5'|| s[i] == '7') ans++;
        else t += s[i];
    }
    
    vector <int> cum(t.size()+2);
    rrep(i, t.size()){
        cum[i] = cum[i+1];
        if(t[i] == '9') cum[i]++;
    }
    int cnt1=0;
    
    s = "";
    rep(i,t.size()){
        if(t[i] == '1') {
            if(cnt1 >= cum[i]) s+= t[i];
            else cnt1++;
        }else{
            if(cnt1>0) cnt1--, ans++;
            else s+= t[i];
        }
    }
    
    cnt1 = 0;
    int cnt9 = 0;
    rep(i, s.size()){
        if(s[i] == '9') cnt9++;
        else cnt1++;
    }
    ans += min(cnt9/2, cnt1);
    cnt1 -= cnt9/2;
    if(cnt1 >1){
        ans += cnt1/2;
    }
    cout<<ans<<endl;
    //cout<<s<<endl;
}