Copy to next blank line

I'm new to macro - so don't laugh... I'm trying to copy Cells A1 and B1 from 400 worksheets to a single worksheet (Merge). The following macro code copies and pastes the cells but overwrites each time the macro is run.

How can I tell the system to paste in the next blank row?


Sub Macro5()
'
' Macro5 Macro

'
Range("A1:B1").Select
Selection.Copy
Sheets("Merged").Select
Range("A1").Select
ActiveSheet.Paste

End Sub