View Compilation Error

0_0_15024908_29599.cpp: In function 'void dft(comp*, int, int)':
0_0_15024908_29599.cpp:42:34: error: 'M_PI' was not declared in this scope
   comp wn = comp(cos(2.0 * inv * M_PI / m), sin(2.0 * inv * M_PI / m));
                                  ^