Problem:
Score: 200 points

Problem Statement

Takahashi bought a piece of apple pie at ABC Confiserie. According to his memory, he paid N yen (the currency of Japan) for it.

The consumption tax rate for foods in this shop is 8 percent. That is, to buy an apple pie priced at X yen before tax, you have to pay X \times 1.08 yen (rounded down to the nearest integer).

Takahashi forgot the price of his apple pie before tax, X, and wants to know it again. Write a program that takes N as input and finds X. We assume X is an integer.

If there are multiple possible values for X, find any one of them. Also, Takahashi's memory of N, the amount he paid, may be incorrect. If no value could be X, report that fact.

Constraints

1 \leq N \leq 50000

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

If there are values that could be X, the price of the apple pie before tax, print any one of them.

If there are multiple such values, printing any one of them will be accepted.

If no value could be X, print :(.

Sample Input 1

432

Sample Output 1

400

If the apple pie is priced at 400 yen before tax, you have to pay 400 \times 1.08 = 432 yen to buy one.

Otherwise, the amount you have to pay will not be 432 yen.

Sample Input 2

1079

Sample Output 2

:(

There is no possible price before tax for which you have to pay 1079 yen with tax.

Sample Input 3

1001

Sample Output 3

927

If the apple pie is priced 927 yen before tax, by rounding down 927 \times 1.08 = 1001.16, you have to pay 1001 yen.

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 waaaa = new waaaa();
        waaaa.Solve();
    }
}

public class waaaa
{
    const int MOD = 1000000007;
    const int INF = 1 << 30;
    const long INFL = 1 << 62;
    const int MOD2 = 998244353;

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

    public void Solve()
    {
        //ここから
        var o = IO.IntM;
        int N = o[0];
        int f=0;
        for (int i = 0; i < N; ++i)
        {
            if(Math.Floor((double)i*1.08)==N)f=i;
        }
        if(f!=0)IO.wri(f);
        else IO.wri(":(");

    }



}

//b-aで降順

public class Math2
{
    public long[] Fact;  //Nまでの階乗
    public long[] invFact;  //階乗の逆元

    //Nまでの階乗をセットしておく
    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;
        }

    }

    //nCr 引数:n r (mod)
    public long nCr(long n, long r, long MOD = 1000000007)
    {
        return (Fact[n] * invFact[r] % MOD * invFact[n - r] % MOD);
    }

    //階乗(O(logN)) 引数:底x n乗　(MOD)
    public long pow(long x, long n, long MOD = 1000000007)
    {
        long res = 1;
        while (n > 0)
        {
            if ((n & 1) != 0) res = res * x % MOD;
            x = x * x % MOD;
            n >>= 1;
        }
        return res;
    }

    //最大公約数 引数:整数x,y
    public long GCD(long x, long y)
    {
        while (y != 0)
        {
            var r = x % y;
            x = y;
            y = r;
        }
        return x;
    }

    //最小公倍数 引数:整数x,y
    public long LCM(long x, long y)
    {
        return x * y / GCD(x, y);
    }

    //素因数分解 引数:整数N
    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;

    }

    //約数列挙
    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;

    }

}

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];

    //out
    public void wri<T>(T x)
    {
        Console.WriteLine(x);
    }

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

}