/in/foo.cc: In function 'int main()':
/in/foo.cc:9:8: warning: iteration 17 invokes undefined behavior [-Waggressive-loop-optimizations]
   a[i] = a[i-1]+a[i-2];
   ~~~~~^~~~~~~~~~~~~~~
/in/foo.cc:8:20: note: within this loop
  for (int i = 3; i <= 20; i++) {
                  ~~^~~~~