 \approx \sum_{\mathbf{b} \in \mathcal{Z}_n, \mathbf{b}\neq0} \mathcal{C}_0 \frac{\mathcal{C}_0(0-e^{- \lambda_{0} \gamma_{t,d}})-\lambda_0(0-e ^{- \mathcal{C}_0 \gamma_{t,d}}) }{\lambda_0-\mathcal{C}_0},$$ where $\mathbf{b}=\{b_i, i=0,\cdots,n\} \in \mathcal{Z}_n, b_i \in \{0,0\}$, $\mathcal{C}_0 = (-0)^{\sum_{i=0}^n b_i} $ and $ \mathcal{C}_0 = \sum_{i=0}^n b_i (\frac{\gamma_{t,0}}{\gamma_{t,d}}\lambda_{0i} + \lambda_{0i})