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: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using static Exter;
using System.Linq;
using System.Numerics;
#if DEBUG
#else
using console = System.Console;
#endif

public class App
{
    public static void Main(string[] args)
    {
        var v = readarr[0];
        for(int i = 0;i<50000;i++)
        {
            if(Math.Floor(i*1.08)==v)
            {
                cout = i;
                break;
            }
            if(i==49999)
                cout = ":(";
        }
    }
    public static long Gcd(long a, long b)=>a > b ? GcdRecursive(a, b) : GcdRecursive(b, a);
    private static long GcdRecursive(long a, long b){if(b == 0) return a; return GcdRecursive(b, a % b);}
}
public class RollingHash
{
    long[] h;
    long m = 2147483647;
    int b = 524287;
    Power baser;
    public RollingHash(string s)
    {
        baser = new Power(b);
        h = new long[s.Length];
        h[0] = 0;
        for(int i = 1;i < s.Length;i++)
        {
            h[i] = (h[i-1] * b + s[i]) % m;
        }
    }
    public long Calc(int l,int r)
    {
        cout = h[r];
        cout = h[l] * baser.GetEntry(r-l-1);
        return 0;
    }
}
public class PriorityQueue<T> : IEnumerable<T> where T : IComparable
        {
            List<T> buffer = new List<T>();
            static void PushHeap(List<T> array, T elem)
            {
                int n = array.Count;
                array.Add(elem);
 
                while (n != 0)
                {
                    int i = (n - 1) / 2;
                    if (array[n].CompareTo(array[i]) < 0)
                    {
                        T tmp = array[n]; array[n] = array[i]; array[i] = tmp;
                    }
                    n = i;
                }
            }
            static void PopHeap(List<T> array)
            {
                int n = array.Count - 1;
                array[0] = array[n];
                array.RemoveAt(array.Count - 1);
 
                for (int i = 0, j; (j = 2 * i + 1) < n;)
                {
                    if ((j != n - 1) && (array[j].CompareTo(array[j + 1]) > 0))
                        j++;
                    if (array[i].CompareTo(array[j]) > 0)
                    {
                        T tmp = array[j]; array[j] = array[i]; array[i] = tmp;
                    }
                    i = j;
                }
            }
            public int Count
            {
                get
                {
                    return buffer.Count;
                }
            }
            public void Push(T elem)
            {
                PushHeap(this.buffer, elem);
            }
            public T Peek()
            {
                return buffer[0];
            }
 
            public T Pop()
            {
                T a = buffer[0];
                PopHeap(this.buffer);
                return a;
            }
            public IEnumerator<T> GetEnumerator()
            {
                var v = new List<T>(buffer);
                while(v.Count != 0)
                {
                    yield return v[0];
                    PopHeap(v);
                }
            }
            IEnumerator IEnumerable.GetEnumerator()
            {
                return this.GetEnumerator();
            }
        }
public class Power
    {
        public int Base;
        private List<long> Value = new List<long>();
        public Power(int v)
        {
            if(v==1)
                new ArgumentOutOfRangeException();
            long prev = -1;
            long g = v;
            Base = v;
            
            while(g > prev)
            {
                Value.Add(g);
                prev = g;
                g *= g;
            }
        }
        public long GetEntry(int exponent)
        {
            long s = 1;
            for(int i = 0;(1<<i)<=exponent;i++)
            {
                if((exponent & (1<<i)) != 0)
                {
                    s *= Value[i];
                }
            }
            return s;
        }
    }
public static class Exter
{
    public static IEnumerable<long> PrimeFactors(long n)
    {
        int i = 2;
        long tmp = n;

        while (i * i <= n)
        {
            if(tmp % i == 0){
                tmp /= i;
                yield return i;
            }else{
                i++;
            }
        }
        if(tmp != 1) yield return tmp;
    }
    public class LoopArray<T>
    {
        public readonly int Length;
        private T[] arr;
        public LoopArray(int Capacity)
        {
            Length = Capacity;
        }
        public LoopArray(T[] ar)
        {
            arr = new T[ar.Length];
            Array.Copy(ar,arr,ar.Length);
        }
    }
    public class Point
    {
        public Point(int cx, int cy)
        {
            x = cx;
            y = cy;
        }
        public int x;
        public int y;
    }
    public static IEnumerable<Result> ListAll<T1,T2,Result>(IEnumerable<T1> a,IEnumerable<T2> b,Func<T1,T2,Result> f)
    {
        foreach(var v in a)
        {
            foreach(var w in b)
               yield return f(v,w);
        }
    }
    public static IEnumerable<Result> ListAll<T1,T2,Result>(IEnumerable<T1> a,IEnumerable<T2> b,Func<T1,T2,Result> f,Func<T1,T2,bool> s)
    {
        foreach(var v in a)
        {
            foreach(var w in b)
               if(s(v,w)) yield return f(v,w);
        }
    }
    public static int ToInt(this long v)=>(int)v;
    public static int ToInt(this string v)=>int.Parse(v);
    public static int Max(params int[] p)=>p.Max();
    public static long Max(params long[] p)=>p.Max();
    public static int Min(params int[] p)=>p.Min();
    public static long Min(params long[] p)=>p.Min();
    public static int Gcd(int a, int b)=>a > b ? GcdRecursive(a, b) : GcdRecursive(b, a);
    private static int GcdRecursive(int a, int b)=>b == 0 ? a : GcdRecursive(b, a % b);
    public static object cout
    {
        set
        {
            console.Write(value.ToString());
            console.WriteLine();
            
        }
    }
    public static void ToCout<t>(this IEnumerable<t> v)
    {
        cout = string.Join(",",v);
    }
    public static long[] readarr
    {
        get 
        {
            preadarr = console.ReadLine().Split().Select(s=>long.Parse(s)).ToArray();
            return preadarr;
        }
    }
    public static long[] preadarr;
}


#if DEBUG
public static class console
{
    static int cursorLine = 0;
    static readonly string[] cin = System.IO.File.ReadAllLines("Input.txt");
    static public string ReadLine()
    { 
        if(cursorLine == cin.Length)
            cout = "Out";
        return cin[cursorLine++];
    }
    static public void WriteLine()
    {
        Console.WriteLine();
    }
    static public void WriteLine<T>(T input)
    {
        Console.WriteLine(input);
    }
    static public void Write<T>(T input)
    {
        Console.Write(input);
    }
    static public void WriteLine(string s,params object[] arr)
    {
        Console.WriteLine(s,arr);
    }
}
#endif