Hi guys, I'm an economics newbie, while running the ARIMA analysis, I have a problem about how to reverse the log first difference value. After I ran:

Code:
 
arima ln_wpi, arima(1,1,4)
predict y_hat
tsline ln_wpi y_hat
I got the figure of the actual value (which is ln_wpi) v.s. the predicted value (which is y_hat), whereas y_hat, is in log first difference form.

How to return the log difference predicted value to the original form in order to compare those?

Many thanks!