#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
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; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N;
	cin >> N;
	string S;
	cin >> S;
	// 11, 19, 991
	int ans = 0, s = 0;
	vector<int> v;
	for (char c : S) {
		const int n = c - '0';
		if (n == 3 || n == 5 || n == 7) ++ans;
		else v.emplace_back(n);
	}
	{
		int l = 0, r = v.size();
		vector<bool> done(v.size());
		while (l < r) {
			while (l < N && v[l] != 1) ++l;
			while (r > 0 && v[r - 1] != 9) --r;
			if (l >= r) break;
			++ans;
			done[l++] = done[--r] = true;
		}
		vector<int> nv;
		REP(i, v.size()) if (!done[i]) nv.emplace_back(v[i]);
		v = nv;
	}
	int one = 0, nine = 0;
	REP(i, v.size()) {
		if (v[i] == 1) {
			if (nine >= 2) { ++ans; nine = -INF; }
			else ++one;
		}
		else ++nine;
	}
	ans += one / 2;
	cout << ans << endl;
}