View Compilation Error

0_0_31071099_32257.cpp: In function 'int main()':
0_0_31071099_32257.cpp:24:22: error: 'M_PI' was not declared in this scope
     printf("%.6lf\n",M_PI*0.5);
                      ^
0_0_31071099_32257.cpp:31:12: error: 'M_PI' was not declared in this scope
   double r=M_PI*0.5;
            ^