#include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int solve(string str) {
	int n = str.size();
	vector<bool> used(n);
	int ans = 0;
	int l = 0, r = n - 1;
	while (true) {
		while (l != n && str[l] == '9') ++l;
		while (r != -1 && str[r] == '1') --r;
		if (l >= r) break;
		used[l] = true, used[r] = true;
		++l; --r;
		++ans;
	}
	l = 0, r = n - 1;
	while (true) {
		while (l != n && (used[l] || str[l] == '1')) ++l;
		++l;
		while (l != n && (used[l] || str[l] == '1')) ++l;
		while (r != -1 && (used[r] || str[r] == '9')) --r;
		if (l >= r) break;
		used[r] = true;
		++ans;
		++l; --r;
	}
	int remc = 0;
	for (int i = 0; i < n; ++i) {
		if (!used[i] && str[i] == '1') ++remc;
	}
	ans += remc / 2;
	return ans;
}
int main() {
	int n;
	string s;
	cin >> n >> s;
	string str;
	int ans = 0;
	for (int i = 0; i < s.size(); ++i) {
		if (s[i] == '1' || s[i] == '9') {
			str += s[i];
		}
		else {
			++ans;
		}
	}
	ans += solve(str);
	cout << ans << endl;
	return 0;
}