#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll inf=1444999999999999;
ll mod=998244353;
int main(){
    ll n;
    cin >> n;
    string s;
    cin >> s;
    ll ans=0;
    ll x=0,y=0;
    for (ll i = 0; i < s.size(); i++)
    {
        if (s[i]=='3'||s[i]=='5'||s[i]=='7')
        {
            ans+=1;
        }else if (s[i]=='1')
        {
            x+=1;
        }else{
            if (x>0)
            {
                ans+=1;
                x-=1;
            }
        }
    }
    ans+=x/2;
    cout << ans << endl;
}