View Compilation Error

0_0_28090518_20319.cpp: In function 'void fft(int, cplx*, cplx*, bool)':
0_0_28090518_20319.cpp:22:25: error: 'M_PI' was not declared in this scope
         cplx w_(cos(2 * M_PI / s), sin(2 * M_PI / s));
                         ^