Reshaping cross sections to panel data

Dear Ms./Mr.,

I need your help urgently in converting the following data set from 1980-1999,

X_Country1 Y_Country1 X_Country2 Y_Country2 X_Country3 Y_Country3

and I am asked to pool it and run regression for it on stata. I am wondering shall I add a year variable? if yes how can I? how can I transfer it from this shape to pool data shape to run a regression?

Thank you in advance for your support

Best,
Rania