View Compilation Error

0_0_21543866_27494.cpp: In function 'int main()':
0_0_21543866_27494.cpp:10:72: error: 'M_PI' was not declared in this scope
         cout<<setiosflags(ios::fixed)<<setprecision(2)<<pow(l, 2.0)/(2*M_PI)<<endl;
                                                                        ^