Excel...Adding Data Cells from multiple workbooks into separate workbook

[Crusty2020NC]

I have 13 workbooks that I am attempting to consolidate into a annual summary.  I am able to import only the first portion of the equation

=SUM('[FLASH REPORT-01-2Jan-29Jan-2017.xlsx]WK1N'!$B$19)

after which I get errors.

=SUM('[FLASH REPORT-01-2Jan-29Jan-2017.xlsx]WK1N'!$B$19),'[FLASH REPORT-02-30Jan-26Feb-2017.xlsx]WK1N'!$B$19)

Can you advise on how to modify this formula?  Once I can add two cells I should be able to figure out how to add the other 11.

[#2]

@Crusty2020NC Sum is used for ranges, you just want to add 2 cells, so use '+' directly:

'[FLASH REPORT-01-2Jan-29Jan-2017.xlsx]WK1N'!$B$19) + '[FLASH REPORT-02-30Jan-26Feb-2017.xlsx]WK1N'!$B$19