#include <bits/stdc++.h>
using namespace std;
int main(){
	int n;
	while(cin>>n){
		int x=(n+1)/1.08;
		int y=x+0.5;
		if((int)(x*1.08)==n)
			cout<<x<<endl;
		else cout<<":("<<endl;
	}
	
	return 0;
} 