when first book = last book, when first book < last book  and when last book < first book.
5 minutes to got it. its very simple;
solution at o(1); AC 0,015

In fact we can solve it without if, just one (!) formula:
abs( ( n2 - n1 ) * ( l1 + 2 * l2 ) - l1 )
8-)

Queen Anne is dead, too.

:D easy :D only 1 formul is needed i did same as  dAFTc0d3r [Yaroslavl SU] :)