#include <bits/stdc++.h>
typedef long long ll;
typedef long double ld;
const int INF=1e9,MOD=1e9+7,ohara=1e6+10;
const ll LINF=1e18;
using namespace std;
    
#define rep(i,n) for(ll (i)=0;(i)<(int)(n);(i)++)
#define rrep(i,a,b) for(ll i=(a);i<(b);i++)
#define rrrep(i,a,b) for(ll i=(a);i>=(b);i--)
#define all(v) (v).begin(), (v).end()
#define Size(n) (n).size()
#define Cout(x) cout<<(x)<<endl
#define doublecout(a) cout<<fixed<<setprecision(15)<<a<<endl;
#define Cerr(x) cerr<<(x)<<endl
#define fi first
#define se second
#define P pair<ll,ll> 
#define m_p make_pair
#define V vector<ll> 
#define U_MAP unordered_map<ll,ll>
#define pq priority_queue<ll>
#define rpq priority_queue<ll,vector<ll>,greater<ll>>
#define p_b push_back
    
ll n,cnt,ans,a,b,d,tmp,tmpp,m,h,w,x,y,sum,pos,k,q;
ld doua;
int dy[]={1,0,-1,0};
int dx[]={0,1,0,-1};
//int dy[]={-1,0,1,-1,1,-1,0,1};
//int dx[]={-1,-1,-1,0,0,1,1,1};
string alph("abcdefghijklmnopqrstuvwxyz"),s,co,s1;
bool fl;
struct edge{int to,cost;};
U_MAP out;
stack<ll> st1,st2;
    
//------ Believe yourself as a genius!!!!!! ------
    
int main(void){
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false);
    
    cin>>n>>s;
    rep(i,Size(s)){
        if(s[i]=='3'||s[i]=='5'||s[i]=='7'){
            ans++;
            out[i]=1;
        }
    }
    //19
    rep(i,Size(s)){
        if(out[i])continue;
        if(s[i]=='1')st1.push(i);
        else{
            if(Size(st1)==0)continue;
            tmp=st1.top();
            st1.pop();
            out[tmp]=1;
            out[i]=1;
            ans++;
        }
    }
    //991
    rep(i,Size(s)){
        if(out[i])continue;
        if(s[i]=='9')st2.push(i);
        else{
            if(Size(st2)<=1)continue;
            tmp=st2.top();
            out[tmp]=1;
            st2.pop();
            tmp=st2.top();
            out[tmp]=1;
            st2.pop();
            out[i]=1;
            ans++;
        }
    }
    rep(i,Size(s)){
        if(out[i])continue;
        if(s[i]=='1')cnt++;
    }
    ans+=cnt/2;
    Cout(ans);
    return 0;
}