View Compilation Error

0_0_38611875_12115.cpp: In function 'double AreaOfOverlap(point, double, point, double)':
0_0_38611875_12115.cpp:7:12: error: 'M_PI' was not declared in this scope
 #define pi M_PI
            ^~~~
0_0_38611875_12115.cpp:32:16: note: in expansion of macro 'pi'
         return pi*r*r;
                ^~