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 Contest148
{
	class Program
	{
		static void Main(string[] args)
		{
			ulong a = ulong.Parse(Console.ReadLine());
			Console.WriteLine(CountZero(Function(a)));
		}

		static ulong Function(ulong n)
		{
			if(n < 2)
			{
				return 1;
			}
			else
			{
				return n * Function(n - 2);
			}
		}

		static ulong CountZero(ulong value)
		{
			string str = value.ToString();
			ulong count = 0;

			for(int i = str.Length - 1; i > 0; i--)
			{
				string s = str.Substring(i, 1);

				if(s == "0")
				{
					count++;
				}
				else
				{
					return count;
				}
			}

			return count;
		}
	}
}