Power Bi Python: How Can I Subtract the Row beginning with Row 121 with Row 1 and give it out in a new Column

Question

Hello I want to Subtract every row starting with row 121 with row 1 and output the result in a new Column.

Row 121 - Row 1 Row 122 - Row 2 Row 123 - Row 3

and so on u get the idea.

How Can I do this?

Table Name: TestTable Column Name: TestColumn Row has to start in Row 121 and has to subtract with Row 1 so the Row difference here is 120 Rows (maybe we can use this in an function?)

Thank you for your helps

I just got errors trying many things in the internet but don´t know what to write here