View Compilation Error

0_0_36021613_19238.cpp: In function 'double fun(double, double, double, double, double, double)':
0_0_36021613_19238.cpp:4:11: error: expected primary-expression before '=' token
 #define PI=acos(-1.0)
           ^
0_0_36021613_19238.cpp:11:59: note: in expansion of macro 'PI'
     else if(d<=fabs(r1-r2)*fabs(r1-r2))return r1<r2?r1*r1*PI:r2*r2*PI;
                                                           ^
0_0_36021613_19238.cpp:4:11: error: expected primary-expression before '=' token
 #define PI=acos(-1.0)
           ^
0_0_36021613_19238.cpp:11:68: note: in expansion of macro 'PI'
     else if(d<=fabs(r1-r2)*fabs(r1-r2))return r1<r2?r1*r1*PI:r2*r2*PI;
                                                                    ^