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.Text.RegularExpressions;

namespace AtCoderBeginnerContest148
{
    class E_DoubleFactorial
    {
        static void Main(string[] args)
        {
            var n = long.Parse(Console.ReadLine());
            var val = F(n).ToString();
            var result = Regex.Match(val, "0+$").Value;
            Console.WriteLine(result.Length);
        }
        static long F(long n)
        {
            if (n < 2)
            {
                return 1;
            }
            return n * F(n - 2);
        }
    }
}