double computeAngle(double x1, double y1, double x2, double y2, double x3, double y3) {
	//A(x1,y1)
	//B(x2,y2)
	//C(x3,y3)
	//AB (x2-x1, y2-y1)
	//AC (x3-x1, y3-y1)
	double cosphi = ((x2-x1)*(x3-x1)+(y2-y1)*(y3-y1)) / sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1)) / sqrt((x3-x1)*(x3-x1)+(y3-y1)*(y3-y1));
	double phi = acos(cosphi);
	if ((x2-x1)*(y3-y1)-(x3-x1)*(y2-y1) < 0) {
		phi = -phi;
	}
	return phi;
}