12. There are $N(N+1) \div 2$ dominoes, each with two numbers, and these numbers are from $1 \sim N$ (the two numbers can be the same). No two dominoes have the same pair of numbers. Now, arrange these dominoes into several "trains," where in each "train," any two adjacent dominoes have the same number on the adjacent sides. The figure below shows a "train" when $N=3$.
When $N=2016$, at least $\qquad$ trains are needed to use up all $2016 \times(2016+1) \div 2$ dominoes.
【Answer】 1008