/in/foo.cc: In function 'int main()':
/in/foo.cc:22:18: warning: iteration 19999u invokes undefined behavior [-Waggressive-loop-optimizations]
      a[i+1]+=a[i];
                  ^
/in/foo.cc:20:15: note: containing loop
  for(int i=1;i<=20001;i++){
               ^