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)

// whileによるループ処理
using System;
public class Program{
    public static void Main(){
        long fn=1;
        var data=Console.ReadLine();
        long n =long.Parse(data);
        long cnt=0;
        if(n%2==1){
            Console.WriteLine(0);
        }else{
            while(n>=0){
               if(n>0){
                 fn=fn*n;  
               } 
                n=n-2;
            }
            while(fn%10==0){
                fn=fn/10;
                cnt++;
            }
            Console.WriteLine(cnt);
        }
    
    }
}