This was a simple sign error, because it's cheaper to save fuel than to buy fuel, as is well known to companies that do it all the time -- for example, Dupont, SD micro electronics.
Jednostavno, imamo pogrešan predznak jer je jeftinije štedjeti gorivo nego ga kupovati, što je dobro poznato kompanijama koje to stalno rade -- na primjer: Dupont, SD Micro Electronics