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)

//ヒントがっつり見た//キャストやめたら成功するようになった
using System;
namespace tst
{
    class Program
    {
        public static long Gusu(long x)
        {
            long cnt = 0;
            long bunbo = 10;
            while (x >= bunbo)
            {
                cnt += x / bunbo;
                bunbo *= 5;
            }
            return cnt;
        }
        /*public static long Kisu(long x)
        {
            long sum = 1;
            for (int i = 1; i <= ((x+1) / 2); i++)
            {
                sum *= ((2 * i)-1);
            }
            return sum;
        }*/
        public static void Main()
        {
            long n = long.Parse(Console.ReadLine());
            if (n % 2 == 1)
            {
                //val = Kisu(n);
                //奇数は0になることがない
                Console.WriteLine(0);
            }
            else if (n % 2 == 0)
            {
                //5の出てくる回数を数えよう
                Console.WriteLine(Gusu(n));
            }
        }

    }
}