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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class Program
{
    public static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        int n = N;
        double ans;

        Sub s = new Sub();
        ans =s.Calc(N);
        int count=0;
        string A = ans.ToString();
        for(int a = A.Length; a>0; a--)
        {
            if (A[a-1] != '0')
                break;
            else
                count += 1;
        }
        Console.WriteLine(count);

    }
}
class Sub
{
    public double Calc(int n)
    {
        double ans=0;
        if (n == 1 | n == 0)
            ans = 1;
        else
        {
            ans = n*Calc(n - 2);
        }
        return ans;
    }
}