#include <bits/stdc++.h>
using namespace std;

#define FOR(i, m, n) for(int (i) = (m); (i) < (n); (i)++)
#define REP(i, n) FOR(i, 0, n)
#define co(n) cout << (n) << endl
#define cosp(n) cout << (n) << ' '
#define all(s) (s).begin(), (s).end()
#define pb push_back
#define mp make_pair
#define fs first
#define sc second
typedef long long ll;
typedef pair<int, int> P;

const ll INF = 2e9;
const ll MOD = 1e9+7;
//const ll MOD 998244353;

int main(void){
	int n;
	cin >> n;

	string arr;
	cin >> arr;

	int ans = 0, num1 = 0, num9 = 0;
	REP(i, n){
		if(arr[i] == '3' || arr[i] == '7') ans++;
		else if(arr[i] == '1') num1++;
		else{
			if(num1){
				ans++;
				num1--;
			}else num9++;
		}
	}

	while(num1 && num9 >= 2){
		ans++;
		num1--;
		num9 -= 2;
	}

	ans += num1/2;

	co(ans);

	return 0;
}