Formula for the number of cycles of a given length in the complete bipartite graph $K_{m,n}$.

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Let $C(m,n;k)$ denote the number of cycles of length $k$ in the complete bipartite graph $K_{m,n}$. Obtain a closed formula for the generating series $$\sum_{m,n,k\geq 0}C(m,n;k)\frac{x^my^nz^k}{m!n!}.$$