#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;
	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 l1 = 0, l9 = 0, r1 = v.size(), r9 = v.size();
	while (l1 < r1 || l9 < r9) {
		while (l1 < r1 && v[l1] != 1) ++l1;
		while (l9 < r9 && v[l9] != 9) ++l9;
		while (l1 < r1 && v[r1 - 1] != 1) --r1;
		while (l9 < r9 && v[r9 - 1] != 9) --r9;
		if (l1 >= r1) break;
		if (l9 >= r9) {
			int one = 0;
			while (l1 < r1) one += v[l1++] == 1;
			ans += one / 2;
			break;
		}
		
		if (l1 < l9) {
			// 11...19*
			++ans;
			++l1;
			++l9;
		}
		else if (r1 < r9) {
			// *19...99
			++ans;
			--r1;
			--r9;
		}
		else {
			// 911...9*1
			++l9;
			while (l9 < r9 && v[l9] != 9) ++l9;
			if (l9 < r9) {
				++ans;
				++l9;
				--r1;
			}
		}
	}
	
	cout << ans << endl;
}