using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using static System.Console;
using static System.Math;

class Program
{
    static void Main()
    {
        var N = int.Parse(ReadLine());
        var S = ReadLine();
        var ans = 0;
        var s = new List<int>();
        for (int i = 0; i < N; i++)
        {
            var n = S[i] - '0';
            if (n == 3 || n == 5 || n == 7) ans++;
            else s.Add(n);
        }
        var c = 0;
        var m = 0;
        for (int i = 0; i < s.Count; i++)
        {
            if (s[i] == 1)
            {
                c++;
            }
            else
            {
                c--;
                m++;
            }
            if (c < 0)
            {
                m--;
                c = 0;
            }
        }
        ans += m;
        var k = 0;
        for (int i = 0; i < s.Count; i++)
        {
            if (s[i] == 1) k++;
        }
        k = Max(k - m, 0)/2;
        ans += k;
        WriteLine(ans);
    }
}