#include <iostream>
#include <string>
#include <algorithm>
#define llint long long

using namespace std;

llint n;
string s;

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n;
	
	llint ans = 0; char c;
	for(int i = 0; i < n; i++){
		cin >> c;
		if(c == '3' || c == '5' || c == '7') ans++;
		else s += c;
	}
	
	llint cnt1 = 0, cnt9 = 0;
	for(int i = 0; i < s.size(); i++){
		if(s[i] == '1') cnt1++;
		else{
			if(cnt1) ans++, cnt1--;
			else cnt9++;
		}
	}
	ans += cnt1/2, cnt1%=2;
	if(cnt9 >= 2 && cnt1 >= 1) ans++;
	
	cout << ans << endl;
	
	return 0;
}