how to convert markdown with sub-markdowns to ebook

Question

main.md contain sub-mds:

main.md

1. [1 - ](1.md)
1. [2 - ](2.md)
1. [3 - ](3.md)
1. [4 - ](4.md)
1. [5 - ](5.md)
1. [6 - ](6.md)

I use calibre command:

ebook-convert main.md main.epub

But only the content of main.md is wrote to epub.