View Compilation Error

0_0_23925649_16646.cpp: In function 'int main()':
0_0_23925649_16646.cpp:4:21: error: 'acos' was not declared in this scope
 #define pi acos(-1.0);
                     ^
0_0_23925649_16646.cpp:26:30: note: in expansion of macro 'pi'
         printf("%.4lf\n",mid*pi);
                              ^
0_0_23925649_16646.cpp:26:32: error: expected primary-expression before ')' token
         printf("%.4lf\n",mid*pi);
                                ^