#include<iostream>
using namespace std;
int main() {
	int n;
	cin >> n;
	for (int i = 1; i <= 50000; i++) {
		int x = i * 1.08;
		if (x == n) {
			cout << i << endl;
			return 0;
		}
	}
	cout << ":(" << endl;
	return 0;
}
