In the plane, there are two unit-radius, tangent circles: $k$ and $k_{1}$. One of their common external tangents is the line $e$. Then we draw the circles $k_{2}, k_{3}, \ldots, k_{n}$ in sequence such that each of them is tangent to $k$, $e$, and the circle with the number one less. What is the radius of the $k_{n}$ circle?