/* -*- coding: utf-8 -*-
 *
 * 910.cc:  No.910 素数部分列 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

/* global variables */

char s[MAX_N + 4];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d%s", &n, s);

  int cnt = 0;
  int n1 = 0, n9 = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == '1')
      n1++;
    else if (s[i] == '9') {
      if (n1 > 0) n1--, cnt++;
      else n9++;
    }
    else
      cnt++;
  }

  int d = min(n9 / 2, n1);
  cnt += d;
  n1 -= d;

  cnt += n1 / 2;

  printf("%d\n", cnt);
  return 0;
}