#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n; cin >> n;
	string s; cin >> s;
	vector<int>v;
	int ans = 0;
	rep(i, s.size()) {
		int  x = s[i] - '0';
		if ((3 == x) || (5 == x) || (7 == x)) {
			ans++;
		}
		else {
			if ((9 == x) && !v.empty() && (1 == v.back())) {
				ans++;
				v.pop_back();
			}
			else {
				v.push_back(x);
			}
		}
	}
	int c9 = 0, c1 = 0;
	rep(i, v.size()) {
		if (1 == v[i])c1++;
		else c9++;
	}
	int x = min(c9 / 2, c1);
	c1 -= x;
	ans += x;
	ans += c1 / 2;
	cout << ans << endl;
	return 0;
}