#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main(){
	int n;
	cin >> n;
	for(int i=1;i<=n;i++) {
		if((int)(i*1.08) == n) {
			cout << i << endl;
			return 0;
		}
	}
	cout << ":(" << endl; 
}