View Compilation Error

0_0_4768933_24521.cpp: In member function `void ConvexHull::init(Point*, int)':
0_0_4768933_24521.cpp:94: error: `M_PI' undeclared (first use this function)
0_0_4768933_24521.cpp:94: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_4768933_24521.cpp: In member function `int ConvexHull::intersect(Point, Point, Point&, Point&)':
0_0_4768933_24521.cpp:105: error: `M_PI' undeclared (first use this function)