#include <iostream>
using namespace std;

int main(void) {
	int n;
	cin >> n;

	for (int i = 1; i <= 50000;i++) {
		if ((int)((float)i*1.08) == n) { 
			cout << i;
			return 0;
		}
	}

	cout << ":(";


	return 0;
}