View Compilation Error

0_0_26845768_12452.cpp: In function 'int main(int, char**)':
0_0_26845768_12452.cpp:2:21: error: 'acos' was not declared in this scope
 #define pi acos(-1.0)
                     ^
0_0_26845768_12452.cpp:11:12: note: in expansion of macro 'pi'
         s2=pi*R;
            ^