import java.io.*;
import java.util.*;
import java.math.*;
public class Main
{
	static long mod=1000000007l;
	static int max=Integer.MAX_VALUE,min=Integer.MIN_VALUE;
	static long maxl=Long.MAX_VALUE,minl=Long.MIN_VALUE;
	static BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
	static StringTokenizer st;
    static int max(int a,int b){return Math.max(a,b);}
    static int min(int a,int b){return Math.min(a,b);}
    static int abs(int a){return Math.abs(a);}
    static long max(long a,long b){return Math.max(a,b);}
    static long min(long a,long b){return Math.min(a,b);}
    static long abs(long a){return Math.abs(a);}
    static int sq(int a){return (int)Math.sqrt(a);}
    static long sq(long a){return (long)Math.sqrt(a);}
    static int ncr(int n,int c,long m)
    {
    	long a=1l;
        for(int x=n-c+1;x<=n;x++)a=((a*x)%m);
        long b=1l;
        for(int x=2;x<=c;x++)b=((b*x)%m);
        long v=(div((int)b,m-2,m)%m);
        return (int)((a*v)%m);
    }
    static int fib(int n) 
    { 
        double p=(1+Math.sqrt(5))/2; 
        return (int)Math.round(Math.pow(p,n)/Math.sqrt(5)); 
    }
    static boolean[] sieve(int n)
    {
    	boolean bo[]=new boolean[n+1];
    	bo[0]=true;bo[1]=true;
    	for(int x=4;x<=n;x+=2)bo[x]=true;
    	for(int x=3;x*x<=n;x+=2)
    	{
    		if(!bo[x])
    		{
    			for(int y=x*x;y<=n;y+=x)bo[y]=true;
    		}
    	}
    	return bo;
    }
    static int gcd(int a,int b)
    {
    	if(b==0)return a;
    	return gcd(b,a%b);
    }
    static long div(long a,long b,long m) 
    {
        long r=1l;
        a%=m;  
        while(b>0)
        {
            if((b&1)==1)r=(r*a)%m; 
            b>>=1;
            a=(a*a)%m;  
        } 
        return r; 
    }
    static int i()throws IOException
    {
    	if(!st.hasMoreTokens()) st=new StringTokenizer(br.readLine());
    	return Integer.parseInt(st.nextToken());
    }
    static long l()throws IOException
    {
    	if(!st.hasMoreTokens())st=new StringTokenizer(br.readLine());
    	return Long.parseLong(st.nextToken());
    }
    static String s()throws IOException
    {
    	if(!st.hasMoreTokens())st=new StringTokenizer(br.readLine());
    	return st.nextToken();
    }
    static double d()throws IOException
    {
    	if(!st.hasMoreTokens())st=new StringTokenizer(br.readLine());
    	return Double.parseDouble(st.nextToken());
    }
    static char c()throws IOException
    {
    	if(!st.hasMoreTokens())st=new StringTokenizer(br.readLine());
    	return st.nextToken().charAt(0);
    }
    static boolean b()throws IOException
    {
    	if(!st.hasMoreTokens())st=new StringTokenizer(br.readLine());
    	return Boolean.parseBoolean(st.nextToken());
    }
    static void p(Object p){System.out.print(p);}
    static void p(String p){System.out.print(p);}
    static void p(int p){System.out.print(p);}
    static void p(double p){System.out.print(p);}
    static void p(long p){System.out.print(p);}
    static void p(char p){System.out.print(p);}
    static void p(boolean p){System.out.print(p);}
    static void pl(Object p){System.out.println(p);}
    static void pl(String p){System.out.println(p);}
    static void pl(int p){System.out.println(p);}
    static void pl(char p){System.out.println(p);}
    static void pl(double p){System.out.println(p);}
    static void pl(long p){System.out.println(p);}
    static void pl(boolean p){System.out.println(p);}
    static void pl(){System.out.println();}
    static int[] ari(int n)throws IOException
    {
    	int ar[]=new int[n];
    	st=new StringTokenizer(br.readLine());
    	for(int x=0;x<n;x++) ar[x]=Integer.parseInt(st.nextToken());
    	return ar;
    }
    static int[][] ari(int n,int m)throws IOException
    {
    	int ar[][]=new int[n][m];
    	for(int x=0;x<n;x++)
    	{
    		st=new StringTokenizer(br.readLine());
    		for(int y=0;y<m;y++)ar[x][y]=Integer.parseInt(st.nextToken());
    	}
    	return ar;
    }
    static long[] arl(int n)throws IOException
    {
    	long ar[]=new long[n];
    	st=new StringTokenizer(br.readLine());
    	for(int x=0;x<n;x++) ar[x]=Long.parseLong(st.nextToken());
    	return ar;
    }
    static long[][] arl(int n,int m)throws IOException
    {
    	long ar[][]=new long[n][m];
    	for(int x=0;x<n;x++)
    	{
    		st=new StringTokenizer(br.readLine());
    		for(int y=0;y<m;y++)ar[x][y]=Long.parseLong(st.nextToken());
    	}
    	return ar;
    }
    static String[] ars(int n)throws IOException
    {
    	String ar[]=new String[n];
    	st=new StringTokenizer(br.readLine());
    	for(int x=0;x<n;x++) ar[x]=st.nextToken();
    	return ar;
    }
    static double[] ard(int n)throws IOException
    {
    	double ar[]=new double[n];
    	st=new StringTokenizer(br.readLine());
    	for(int x=0;x<n;x++)ar[x]=Double.parseDouble(st.nextToken());
    	return ar;
    }
    static double[][] ard(int n,int m)throws IOException
    {
    	double ar[][]=new double[n][m];
    	for(int x=0;x<n;x++)
    	{
    		st=new StringTokenizer(br.readLine());
    		for(int y=0;y<m;y++)ar[x][y]=Double.parseDouble(st.nextToken());
    	}
    	return ar;
    }
    static char[] arc(int n)throws IOException
    {
    	char ar[]=new char[n];
    	st=new StringTokenizer(br.readLine());
    	for(int x=0;x<n;x++)ar[x]=st.nextToken().charAt(0);
    	return ar;
    }
    static char[][] arc(int n,int m)throws IOException
    {
    	char ar[][]=new char[n][m];
    	for(int x=0;x<n;x++)
    	{
    		String s=br.readLine();
    		for(int y=0;y<m;y++)ar[x][y]=s.charAt(y);
    	}
    	return ar;
    }
    static void p(int ar[])
    {
    	StringBuilder sb=new StringBuilder("");;
    	for(int a:ar) sb.append(a+" ");
    	System.out.println(sb);
    }
    static void p(int ar[][])
    {
    	StringBuilder sb;
    	for(int a[]:ar)
    	{
    		sb=new StringBuilder("");
    		for(int aa:a) sb.append(aa+" ");
    		System.out.println(sb);
    	}
    }
    static void p(long ar[])
    {
    	StringBuilder sb=new StringBuilder("");
    	for(long a:ar)sb.append(a+" ");
    	System.out.println(sb);
    }
    static void p(long ar[][])
    {
    	StringBuilder sb;
    	for(long a[]:ar)
    	{
    		sb=new StringBuilder("");
    		for(long aa:a)sb.append(aa+" ");
    		System.out.println(sb);
    	}
    }
    static void p(String ar[])
    {
    	StringBuilder sb=new StringBuilder("");
    	for(String a:ar)sb.append(a+" ");
    	System.out.println(sb);
    }
    static void p(double ar[])
    {
    	StringBuilder sb=new StringBuilder("");
    	for(double a:ar)sb.append(a+" ");
    	System.out.println(sb);
    }
    static void p(double ar[][])
    {
    	StringBuilder sb;
    	for(double a[]:ar)
    	{
    		sb=new StringBuilder("");
    		for(double aa:a)sb.append(aa+" ");
    		System.out.println(sb);
    	}
    }
    static void p(char ar[])
    {
    	StringBuilder sb=new StringBuilder("");
		for(char aa:ar)sb.append(aa+" ");
		System.out.println(sb);
    }
    static void p(char ar[][])
    {
    	StringBuilder sb;
    	for(char a[]:ar)
    	{
    		sb=new StringBuilder("");
    		for(char aa:a)sb.append(aa+" ");
    		System.out.println(sb);
    	}
    }
    static public void main(String[] args)throws Exception
    {
    	st=new StringTokenizer(br.readLine());
    	int n=i();
    	for(int x=1;x<=50000;x++)
    	{
    		double d=1.0*x*1.08;
    		if((int)d==n) {
    			pl(x);
    			System.exit(0);
    		}
    	}
    	pl(":(");
    }
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
    	
}


Measured memory_bytes: 21204