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.Linq;
using System.Collections.Generic;
using System.Numerics;
namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var N = long.Parse(Console.ReadLine());
            long sum = 0;
            long i = 1;
            if(N%2 == 0)
            {
                while (true)
                {
                    if (N / (long)Math.Pow(5, i) > 0)
                    {
                        sum += N / (long)Math.Pow(5, i) / 2;
                    }
                    else
                    {
                        break;
                    }
                    i++;
                }
                Console.WriteLine(sum);
            }
            else
            {
                Console.WriteLine("0");
            }
            

            

        }
        
    }

}