import std.stdio;
import std.conv;
import std.math;

void main() {
	double n;
	scanf("%lf", &n);
	auto y = n / 1.08;
	auto n2 = n.to!int;
	auto y2 = y.ceil.to!int;

	if (floor(y2 * 1.08).to!int == n2 || floor((y2+1) * 1.08).to!int == n2) {
		y2.write;
	}
	else {
		":(".write;
	}
}
