Difference between two rows in Spark dataframe

I created a dataframe in Spark, by groupby column1 and date and calculated the amount.

val table = df1.groupBy($"column1",$"date").sum("amount")

Column1 |Date   |Amount
A   |1-jul  |1000
A   |1-june |2000
A   |1-May  |2000
A   |1-dec  |3000
A   |1-Nov  |2000
B   |1-jul  |100
B   |1-june |300    
B   |1-May  |400
B   |1-dec  |300

Now, I want to add new column, with difference between amount of any two dates from the table.