#include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int solve(string str) {
	int n = str.size();
	vector<int> c1(n + 1), c9(n + 1);
	for (int i = 0; i < n; ++i) {
		c1[i + 1] = c1[i] + (str[i] == '1');
		c9[i + 1] = c9[i] + (str[i] == '9');
	}
	int ans = 0;
	for (int i = 0; i <= n; ++i) {
		int l1 = c1[i], r1 = c1[n] - c1[1];
		int l9 = c9[i], r9 = c9[n] - c9[i];
		int pa = min(l1, r9);
		int pb = min(l9 / 2, r1);
		int sub = pa + pb + (c1[n] - pa - pb) / 2;
		ans = max(ans, sub);
	}
	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;
}