/in/foo.cc: In function 'int main()':
/in/foo.cc:11:14: warning: iteration 3 invokes undefined behavior [-Waggressive-loop-optimizations]
   sum+=a[i][j];
        ~~~~~~^
/in/foo.cc:8:15: note: within this loop
  for(int j=1;j<=4;j++)
              ~^~~