/in/foo.cc: In function 'int main()':
/in/foo.cc:24:7: warning: iteration 26 invokes undefined behavior [-Waggressive-loop-optimizations]
   a[i]=-1;
   ~~~~^~~
/in/foo.cc:23:15: note: within this loop
  for(int i=1;i<=27;i++){
              ~^~~~