View Compilation Error

1_866_2392_13169.cpp: In function 'int main()':
1_866_2392_13169.cpp:13:19: error: 'M_PI' was not declared in this scope
   ans = 0.5*sin(2*M_PI/n)*n +(1 - cos(M_PI/n))*sin(M_PI/n);
                   ^