View Compilation Error

0_0_16119492_26400.cpp: In function 'int main()':
0_0_16119492_26400.cpp:29:50: error: 'mid' was not declared in this scope
       sum += acos((sqr(a[j]) + sqr(a[j+1]) - sqr(mid)) / 2 / a[j] / a[j+1]);
                                                  ^
0_0_16119492_26400.cpp:7:18: note: in definition of macro 'sqr'
 #define sqr(x) ((x)*(x))
                  ^