#include<iostream>
using namespace std;
int main()
{
	int n;
	cin>>n;
	int a=n/1.08;
	for(int i=a-1;i<=a+1;i++)
	{
		if((int)(i*1.08)==n)
		{
			cout<<i<<endl;
			return 0;
		}
	}
	
		cout<<":("<<endl;
	
	return 0;
}