64. Let the four vertices of a regular tetrahedron be $A B C D$, with each edge length being 1 meter. A small insect starts from point $A$ and moves according to the following rule: at each vertex, it randomly selects one of the three edges passing through that vertex and crawls all the way to the end of that edge. What is the probability that after crawling 7 meters, it is exactly at vertex $A$? $\qquad$