My objective is to make an "album" of about 50 slides, containing many digital photographs. Once I get past about 25 slides the file size gets too big and unwieldy - like close to 100 MBytes. By dividing the album into two files I can keep the size of each file to a more manageable level. But to be contiguous the second file would have to start with slide 26 - which is the problem. Using a master slide, Impress insists on starting each file with slide number 1.

In my younger, working days, my employer required that I use Microsoft Windows and Powerpoint. In Powerpoint, on the slide setup form there is an entry field for specifying the starting slide number. The default is 1, but it can be changed just by entering the desired number.

Now, in my older and wiser retired days, I avoid all things Microsoft and stick to Linux and Open Office. But not being able to specify a starting slide number in Impress is a deficiency. 

I am aware that I can change the slide name, but the number printed as the page number always starts with 1.