Let $ n$ be a positive integer, and let $ M = \{1,2,\ldots, 2n\}$. Find the minimal positive integer $ m$, such that no matter how we choose the subsets $ A_i \subset M$, $ 1\leq i\leq m$, with the properties: (1) $ |A_i-A_j|\geq 1$, for all $ i\neq j$, (2) $ \bigcup_{i=1}^m A_i = M$, we can always find two subsets $ A_k$ and $ A_l$ such that $ A_k \cup A_l = M$ (here $ |X|$ represents the number of elements in the set $ X$.)