#pragma once

#include <bits/stdc++.h>

#define M_PI       3.14159265358979323846   // pi

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> VI;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, char, ll> t4;

#define rep(a,n) for(int a = 0;a < n;a++)
#define repi(a,b,n) for(int a = b;a < n;a++)

const ll mod = 1000000007;

int main(void)
{
	ll n;
	cin >> n;
	string s;
	cin >> s;
	vector<int> one;
	vector<int> nine;
	ll count = 0;
	rep(i, n) {
		int d = s[i] - '0';
		if (d == 3) count++;
		else if (d == 5) count++;
		else if (d == 7) count++;
		else if (d == 1) one.push_back(i);
		else if (d == 9) nine.push_back(i);
	}
	int k = 0;
	for (int i = 0; i < one.size(); i++) {
		int id = one[i];
		bool found = false;
		while (k < nine.size()) {
			if (nine[k] <= id) {
				k++;
			}
			else {
				found = true;
				break;
			}
		}
		if (!found) {
			count += (one.size() - i) / 2;
			break;
		}
		k++;
		count++;
	}
	cout << count << endl;
	return 0;
}