Copy Rows if first cell matches criteria

Hi everybody,

I am trying to copy a report together in a result worksheet which is based on two input worksheets.

What I have is Input_Worksheet1 which could look like

RP_ID | ER_ID | Num
rp1 | er1 | 123
rp2 | er2 | 321
rp1 | er3 | 99

Input_Worksheet2:

RP_ID | Num
rp1 | 222
rp2 | 321

The Output I am trying to achieve is

RP_ID | ER_ID | Num
rp1 | sum | 222
rp1 | er1 | 123
rp1 | er3 | 99
rp2 | sum | 321
rp2 | er2 | 321


So I am trying to create a function which
1) goes to input worksheet 2
1a) copies the first row to the output worksheet
1b) saves the value of the first cell (in column A) in a temp variable "criterion"
2) goes to input worksheet 1
2b) copies all rows to the output worksheet
2b_?) where the first cell of the row is equal to the saved variable "criterion"
3) Starts over at (1) with the 2nd row of that input worksheet

The code I have found for copying stuff is not really working with dynamic criteria and I can't seem to be able to adapt it fittingly.

Does any of you have suggestions on how to tackle this?

Thank you very much!

Best regards,
Andreas