The figures $G_1,G_2,G_3,\dots$ are constructed from diamonds. $G_1$ is a single diamond. For $n\ge 2$, $G_n$ is constructed from $G_{n-1}$ by expanding it so that the new figure forms a complete square of diamonds, and the outermost square of $G_n$ has two more diamonds on each side than the outermost square of $G_{n-1}$. How many diamonds are there in figure $G_{15}$?