using System;

class Program
{
  Scanner cin;

  int N;
  string S;

  void myon()
  {
    cin = new Scanner();

    N = cin.nextInt();
    S = cin.next();

    int one = 0, nine = 0, other = 0;
    for(int i = 0; i < N; i++)
    {
      if(S[i] == '3' || S[i] == '5' || S[i] == '7')
      {
        other += 1;
      } else if(S[i] == '1')
      {
        one += 1;
      } else
      {
        if(one > 0)
        {
          one -= 1;
          other += 1;
        } else
        {
          nine += 1;
        }
      }
    }

    int make = Math.Min(one, nine / 2);

    Console.WriteLine(other + make + (one - make) / 2);
  }

  static void Main(string[] args)
  {
    new Program().myon();
  }
}

class Scanner
{
  string[] s;
  int i;
  readonly char[] cs = new char[] { ' ' };

  public Scanner()
  {
    s = new string[0];
    i = 0;
  }

  public string next()
  {
    if (i < s.Length) return s[i++];
    string st = Console.ReadLine();
    while (st == "") st = Console.ReadLine();
    s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
    if (s.Length == 0) return next();
    i = 0;
    return s[i++];
  }

  public string nextLine()
  {
    return Console.ReadLine();
  }

  public int nextInt()
  {
    return int.Parse(next());
  }

  public long nextLong()
  {
    return long.Parse(next());
  }

  public double nextDouble()
  {
    return double.Parse(next());
  }
}