using System;

namespace TrailingZerosInN
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            long n = long.Parse(Console.ReadLine());
            long countZero = 0;

            for (int i = 1; i <= n; i++)
            {
                if (i % 5 == 0)
                {
                    countZero += GetFive(i);
                }
            }

            Console.WriteLine(countZero);
        }


        static int GetFive(long number)
        {
            int count = 0;
            while (number % 5 == 0)
            {
                number = number / 5;
                count++;
            }
            return count++;
        }
    }
}