-i flag just defines an infiles. You need to define what you are going to do with those infiles, rather than simply exporting them into your outfile. In view of that, according to ffmpeg documentation, the best method is to start by creating a file with your video in-files in it. Here's an example (just call the file
#this is a comment
Then you can run the following command:
ffmpeg -f concat -safe 0 -i mylist.txt -c copy output
...where output is your output filename.
Two more things bear mentioning. Firstly, the
-safe 0 option is not required if the paths are relative. Secondly, you are not just doing a straight copy in your command mentioned. In other words, your outfile looks like it is supposed to be a different format than your infiles. Therefore, you will have to add on the appropriate flags for the
-c option (codec) to come up with the codecs you are looking for.