#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_);
	
	std::vector<int> pos9, pos1;
	for (int i = (int) s.size() - 1; i >= 0; i--) {
		if (s[i] == '9') pos9.push_back(i);
		else {
			if (pos9.size()) s[pos9.back()] = '0', pos9.pop_back(), res++;
			else pos1.push_back(i);
		}
	}
	
	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;
}