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: Wrong Answer)

using System;

namespace abc148_e
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = double.Parse(Console.ReadLine());
            decimal R = 0;
            double i = 1;
            double r = 1;
            
            if (N % 2 == 0)
            {
                while (r != 0)
                {
                    r = Math.Floor(N / 2 / Math.Pow(5, i++));
                    R += (decimal)r;
                }
                Console.WriteLine(R);
            }
            else
                Console.WriteLine(0);
        }
    }
}