In this article is given the formula for radius of circle $K_{n}$, where in sequence $\{K_{j}\}$, four circles $K_{1}$, $K_{2}$, $K_{n}$, $K_{n+1}$, for all $n\geq 3$, are mutually tangent. Radius $r_{n}$ is expressed in terms of radii $r_{1}$, $r_{2}$, $r_{3}$.