Add Formula Via Macro On Changing Range

I’m wondering if it possible to have a variable in a Formula. I was looking around, but non of the other examples I found was working. The code I was trying is:

Sub All()
Dim lngDataRows As Long
Dim lngDataRowsSum As Long
' add sum
lngDataRowsSum = Range("A5").End(xlDown).Row
Range("A" & lngDataRowsSum).Offset(1, 0).FormulaR1C1 = "Sum"
Range("M" & lngDataRowsSum).Offset(1, 0).FormulaR1C1 = "=sum(M6:M & lngdatarowssum)"
End Sub

Another code I tried was from this thread

Sub All()
Dim lngRowsBottom As Long
Dim lngRowsTop As Long
' add sum
lngDataRowsSum = Range("A5").End(xlDown).Row
Range("C10").formula = "=sum(R["&lngrowstop&"]C:R["&lngrowsbottom&"]C)"
End Sub

The error message is: “Compile error: Expected: enf of statement” and the part “[C:R[“ is highlighted. Tried various ways to solve it but nothing was working.
Does anyone of you know how to get it up and running?