include

using namespace std; int main() {

double a, b ;
cin >> a ;
if (a <= 15) {
	printf("%.2lf", 4 * a / 3);
	return 0;
} else if (a > 15) {
	printf("%.2lf", 2.5 * a - 10.5);
	return 0;
}
return 0;

}