#include <iostream>
using namespace std;

int main() {
	int n; cin >> n;
	for (int i = 1; i <= 50000; ++i) {
		if ((int)(i * 1.08) == n) {
			cout << i << endl;
			return 0;
		}
	}
	cout << ":(" << endl;

	return 0;
}