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You recover the original series $q_k$ by summing the predicted differences $Z_t$ and adding them to the last known actual value $q_l$ (or $q_1$ depending on the starting index). The formula is $q_k = \sum_{i=1}^{k-l} Z_{k-i} + q_l$.

Conditions: You have forecasts for the differenced series $Z_t$.; You know the last observed value of the original series $q_l$.; The relationship $Z_t = q_{t+1} - q_t$ holds.

You recover the original series $q_k$ by summing the predicted differences $Z_t$ and adding them to the last known actual value $q_l$ (or $q_1$ depending on the starting index). The formula is $q_k = \sum_{i=1}^{k-l} Z_{k-i} + q_l$.

Conditions: You have forecasts for the differenced series $Z_t$.; You know the last observed value of the original series $q_l$.; The relationship $Z_t = q_{t+1} - q_t$ holds.