Let $k$ be a positive integer and $a_1, a_2,$ $\cdot \cdot \cdot$ $, a_k$ digits. Prove that there exists a positive integer $n$ such that the last $2k$ digits of $2^n$ are, in the following order, $a_1, a_2,$ $\cdot \cdot \cdot$ $, a_k , b_1, b_2,$ $\cdot \cdot \cdot$ $, b_k$, for certain digits $b_1, b_2,$ $\cdot \cdot \cdot$ $, b_k$