IMO 1998 - Shortlisted Problems Question 21

(CAN) Let $a_0,a_1,a_2,\ldots$ be an increasing sequence of nonnegative integers such that every nonnegative integer can be expressed uniquely in the form $a_i+2a_j+4a_k$, where $i,j,k$ are not necessarily distinct. Determine $a_{1998}$.