#include<iostream>
#include <cmath>
using namespace std;
int main(){
	int N,X;
	cin>>N;
	X=(int)(N/1.08);
	for(int i=X-1;i<=X+1;i++){
		if(floor(i*1.08)==N){cout<<i;return 0;}
	}
	
	cout<<":(";
	return 0;
}