#include <stdio.h>
int main()
{
	double x, y;
	scanf("%lf", &x);
	if (x <= 15)
	{
		y = 4 * x / 3;
	}
	else
	{
		y = 2.5 * x - 17.5;
	}

	printf("%.2f\n", y);
	
	return 0;
}