file path: reference to other workbooks

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The file links to different workbooks by conditions, I can get the file
name, path, etc,

such as:

in A1: ="c:\abc\"&lastmonth&".xls" --> c:\abc\june.xls

or A1: ="c:\abc\"&"["lastmonth&".xls]" --> c:\abc\[june.xls]

I name A1 as LastFile, how can I use it to reference a cell in that
file? like

=c:\abc\[june.xls]total!a1

I tried a few combinations but didn't work, what is the correct format?
Thank you.

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