#include <bits/stdc++.h>

using namespace std;

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