$$ t_1^H = \Bigg(-\frac{1}{\lambda}\log{\bigg( \sum_{n=1}^{n} \Big(\frac{e^{\lambda(t_j^L-1)^\beta}}{(1 + \rho)^n} \Big)  - \sum_{i=2}^{m} \Big(\frac{e^{- \lambda(t_i^H-1)^\beta}}{(1 + \rho)^n} \Big) \bigg)\Bigg)^\frac{1}{\beta}} + 1$$