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;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        var wrong_answer = new wrong_answer();
        wrong_answer.Solve();
    }
}

public class wrong_answer
{
    const int INF = 1 << 30;
    const long INFL = 1L << 60;
    const int MOD = 1000000007;
    const int MOD2 = 998244353;

    myinout IO = new myinout();
    Math2 mmath = new Math2();


    public void Solve()
    {
        //ここから
        var o = IO.longm;
        var n = o[0];
        if(n==0)IO.wri(1);
        else if((n%2)==1)IO.wri(0);
        else{
            var k = n / 2;
            long aaa = 0;
            for (long i = 5; i < k;i*=5){
                aaa += (k / i);
            }
            var ans = Math.Min(k, aaa);
            IO.wri(ans);
        }

    }
    
}




//b-aで降順

public class Math2
{
    public long[] fact;
    public long[] invfact;
    public void factoricalmodset(long N, long MOD = 1000000007)
    {
        fact = new long[N + 1];
        invfact = new long[N + 1];
        fact[0] = 1;

        for (long i = 1; i <= N; ++i)
        {
            fact[i] = fact[i - 1] * i % MOD;
        }
        invfact[N] = pow(fact[N], MOD - 2);

        for (long i = N; i > 0; --i)
        {
            invfact[i - 1] = invfact[i] * i % MOD;
        }

    }

    public long nCr(long n, long r, long MOD = 1000000007)
    {
        return (fact[n] * invfact[r] % MOD * invfact[n - r] % MOD);
    }

    public long pow(long i, long n, long MOD = 1000000007)
    {
        long res = 1;
        while (n > 0)
        {
            if ((n & 1) != 0) res = res * i % MOD;
            i = i * i % MOD;
            n >>= 1;
        }
        return res;
    }

    public long gcd(long i, long y)
    {
        while (y != 0)
        {
            var r = i % y;
            i = y;
            y = r;
        }
        return i;
    }

    public long lcm(long i, long y)
    {
        return i * y / gcd(i, y);
    }

    public Dictionary<long, int> primefactorization(long N)
    {
        var ret = new Dictionary<long, int>();

        for (long i = 2; i * i <= N; ++i)
        {
            int cnt = 0;
            while (N % i == 0)
            {
                cnt++;
                N /= i;
            }
            if (cnt != 0) ret[i] = cnt;

        }
        if (N >= 2) ret[N] = 1;

        return ret;

    }

    public List<long> divisorenumrate(long N)
    {
        var ret = new List<long>();

        for (long i = 1; i * i <= N; ++i)
        {

            if (N % i == 0)
            {
                ret.Add(i);
                ret.Add(N / i);
            }

        }

        return ret;

    }

    public void swap<T>(ref T a, ref T b)
    {
        var i = a;
        a = b;
        b = i;
    }

}

class myinout
{
    //in
    public string str => Console.ReadLine();

    public string[] strm => str.Split(' ');

    public long[] longm => strm.Select(long.Parse).ToArray();
    public int[] intm => strm.Select(int.Parse).ToArray();
    public char[] charm => str.ToArray();
    public long long1 => longm[0];
    public int int1 => intm[0];
    public char char1 => charm[0];

    //out
    public void wri<T>(T i)
    {
        Console.WriteLine(i);
    }
    public void wri<T>(T[] i)
    {
        Console.WriteLine(string.Join(" ", i));
    }

    public void yn(bool i)
    {
        if (i) wri("Yes");
        else wri("No");
    }

}