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;
using System.Collections.Generic;
using System.Linq;

namespace AtCoderBeginnerContest148
{
	class Program
	{
		static void Main(string[] args)
		{
			var input = long.Parse(Console.ReadLine());
			if (input % 2 == 1)
			{
				Console.WriteLine("0");
				return;
			}

			long result = 0;
			var index = 1;
			while (input > 0)
			{
				Console.WriteLine((long)(input / 10));
				result += (long)(input / 10);
				index++;
				input /= 5;
			}

			Console.WriteLine(result);
		}
	}
}