The exact value of $c_1(K_{2,n})$

For a graph $G$, let $c_1(G)$ be the largest distortion necessary to embed any shortest-path metric on $G$ into ${\mathrm{\ell }}_1$, and for any natural number $n,m\in \mathbb{N}$, denote $K_{n,m}$ as the complete bipartite graph. In this note, we caculate the value of $c_1(K_{2,n})$, more precisely we prove $c_1(K_{2,n})=\frac{3k-2}{2k-1}$ where $k=\lceil \frac{n}{2}\rceil$.