include <stdio.h>

int main() {

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

}