Problem:
Score : 500 points

Problem Statement

For an integer n not less than 0, let us define f(n) as follows:

f(n) = 1 (if n < 2)

f(n) = n f(n-2) (if n \geq 2)

Given is an integer N. Find the number of trailing zeros in the decimal notation of f(N).

Constraints

0 \leq N \leq 10^{18}

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of trailing zeros in the decimal notation of f(N).

Sample Input 1

12

Sample Output 1

1

f(12) = 12 × 10 × 8 × 6 × 4 × 2 = 46080, which has one trailing zero.

Sample Input 2

5

Sample Output 2

0

f(5) = 5 × 3 × 1 = 15, which has no trailing zeros.

Sample Input 3

1000000000000000000

Sample Output 3

124999999999999995

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

internal static class CR
{
    public static string Get() { return Console.ReadLine(); }
    public static long GetInteger() { return long.Parse(Console.ReadLine()); }
    public static IEnumerable<ulong> GetIntegers() { return Console.ReadLine().Split(' ').Select(x => ulong.Parse(x)); }
}

namespace AtCoder.E
{
    public class Program
    {
        public static long N { get; private set; }

        public static void Main() { var r = GetResult(); Debug.WriteLine(r); Console.WriteLine(r); }

        private static object GetResult()
        {
            N = CR.GetInteger();

            // 奇数の時点でNG
            if (N % 2 != 0) return 0;

            var fize = 0L;
            var under = 2L;
            while (under <= N)
            {
                under *= 5;
                //fize = (long)Math.Floor((double)N / under); // NG
                fize += N / under;
            }

            return fize;
        }
    }
}