#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}

int main() {
	int n = ri();
	std::string s;
	std::cin >> s;
	int res = 0;
	std::string s_;
	for (int i = 0; i < n; i++) {
		if (s[i] == '3' || s[i] == '5' || s[i] == '7') res++;
		else s_.push_back(s[i]);
	}
	std::swap(s, s_);
	
	int l = 0;
	int r = (int) s.size() - 1;
	while (l < r) {
		while (l < (int) s.size() && s[l] != '1') l++;
		while (r >= 0 && s[r] != '9') r--;
		if (r <= l) break;
		s[l] = '0';
		s[r] = '0';
		l++, r--;
		res++;
	}
	s_.clear();
	std::vector<int> pos1, pos9;
	for (int i = 0; i < (int) s.size(); i++) {
		if (s[i] != '0') {
			s_.push_back(s[i]);
			if (s[i] == '9') pos9.push_back(i);
			else pos1.push_back(i);
		}
	}
	std::swap(s_, s);
	for (int i = (int) pos9.size() - 2; i >= 0; i--) {
		int pos = pos9[i + 1];
		if (pos1.size() && pos1.back() > pos) pos1.pop_back(), i--, res++;
	}
	res += pos1.size() / 2;
	std::cout << res << std::endl;
	
	
	return 0;
}