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;

namespace E___Double_Factorial
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = long.Parse(Console.ReadLine().Trim());
            Console.WriteLine(Solve(N));

        }
        static long Solve(long N)
        {
            if (N == 0 || N % 2 == 1)
            {
                return 0L;
            }
            else
            {
                long ans = 0;
                long a = 10;
                while (N >= a)
                {
                    ans += N / a;
                    a *= 5;
                }
                return ans;
            }
        }
    }
}