using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Security.Cryptography;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var s = ReadLine();
        var one = 0;
        var nine = 0;
        var ans = 0;
        foreach (var c in s)
        {
            if (c == '3' || c == '5' || c == '7') ++ans;
            else if (c == '1')
            {
                if (nine > 1)
                {
                    ++ans;
                    nine -= 2;
                }
                else ++one;
            }
            else
            {
                if (one > 0)
                {
                    ++ans;
                    --one;
                }
                else ++nine;
            }
        }
        WriteLine(ans + one / 2);
    }
}