Place The Number

Chef has three empty stacks and $N$ numbers from $1$ to $N$. Each number is randomly placed on **one** of the stacks with equal probability of $\frac{1}{3}$ for each stack. The numbers are placed in ascending order, meaning that if $x \lt y$, then $x$ must be placed on a stack before $y$ is placed. Find the probability that the **sum** of the **top** elements in the first **two** stacks is eq