View Compilation Error

0_0_37148797_5838.cpp:11:19: error: expected initializer before 'acos'
 const double M_PI acos(-1);
                   ^
0_0_37148797_5838.cpp: In function 'void FFT(std::complex<double>*, int, int)':
0_0_37148797_5838.cpp:48:26: error: 'M_PI' was not declared in this scope
     cp x(1), cnt(cos(2 * M_PI / n), sin(op * 2 * M_PI / n));
                          ^
0_0_37148797_5838.cpp: In function 'void fft(std::complex<double>*, int, int)':
0_0_37148797_5838.cpp:64:24: error: 'M_PI' was not declared in this scope
         cp cnt(cos(2 * M_PI / i), sin(op * 2 * M_PI / i));
                        ^