In which fileshould I write style sheet(less) in roots starter theme of wordpress?

I am new in roots starter theme of wordpress. In 'assets/less' folder, where should I write style sheet(There I found 4 files - _bootstrap.less,_global.less,main.less,_variables.less).Please anybody explain?

main.less is the file to edit. The files with an _ prefixed are either dynamically generated or are static libraries which should not be amended.

You should use @import within main.less to add in additional code.