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;
class Program
 {
        static void Main(string[] args)
        {
          ulong input =ulong.Parse(Console.ReadLine());
          ulong sum = 0;
          if(input%2==1){
          	input = 0;
          }
          for(int i=1;i<27;i++){
            ulong n = (ulong)System.Math.Pow(5,i)*2;
            decimal p = input/n;
            sum += (ulong)System.Math.Floor(p);
          }
          Console.WriteLine(sum);
        }
 }