The reconstruction of tau-pair production, $e^{+}e^{-} to tau^{+}tau^{-}$, from the subsequent 3-prong ($tau^{+} rightarrow pi^{+} pi^{-} pi^{+} bar{ u}_{tau}$) and 1-prong ($tau^{-} to ell^{-} bar{ u}_{ell} u_{tau}$, $tau^{-} to h^{-} u_{tau}$ or $tau^{-} to pi^{-} pi^0 u_{tau}$) decays, is presented using 8.8 fb$^{-1}$ of $e^{+}e^{-}$ collision data of Belle II at the center-of-mass energy $sqrt{s} = m_{Upsilon(4S)}$. The pseudomass technique developed by the ARGUS experiment is used to measure the $tau$-lepton mass $m_{tau}$ in the 3-prong $tau^{+} to pi^{+} pi^{-} pi^{+} bar{ u}_{tau} $ decay, resulting in $m_{tau} = 1777.28 pm 0.75~{rm (stat.)} pm 0.33 ~{rm (sys.)}~{rm{MeV}/rm{c}^2}$.
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