View Compilation Error

0_0_30751917_22993.cpp: In function 'int main()':
0_0_30751917_22993.cpp:17:36: error: 'M_PI_2' was not declared in this scope
             double k1 = xL == xi ? M_PI_2 // 90
                                    ^
0_0_30751917_22993.cpp:18:66: error: 'M_PI' was not declared in this scope
                                  : atan((yL - yi) / (xL - xi)) + M_PI;
                                                                  ^