/in/foo.cc: In function 'int main()':
/in/foo.cc:26:19: warning: iteration 16 invokes undefined behavior [-Waggressive-loop-optimizations]
   rup[i] = lup[i] = 1;
            ~~~~~~~^~~
/in/foo.cc:25:15: note: within this loop
  for(i  =1; i <=2*N+1; i++)
             ~~^~~~~~~