//カタラン数を語らん!w

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;
typedef string::const_iterator State;
#define Ma_PI 3.141592653589793
#define eps 1e-5
#define LONG_INF 2000000000000000000LL
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL
#define MOD 998244353LL
#define seg_size 262144*2
#define REP(a,b) for(long long a = 0;a < b;++a)
int main() {
	int n;
	cin >> n;
	string s;
	cin >> s;
	int cnt_one = 0;
	int ans = 0;
	for (int i = 0; i < s.length(); ++i) {
		if (s[i] == '1') {
			cnt_one++;
		}
		else if (s[i] == '9') {
			if (cnt_one >= 1) {
				ans++;
				cnt_one--;
			}
		}
		else {
			ans++;
		}
	}
	cout << ans+cnt_one/2 << endl;
}