/in/foo.cc:3:11: error: 'i' was not declared in this scope
    3 | int m,n,a[i],sum;
      |           ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:12:30: error: 'a' was not declared in this scope
   12 |                         cin>>a[i];
      |                              ^