E:\cs>youtube-dl --merge-output-format mp4 --format bestvideo[ext=mp4]+bestaudio[ext=m4a] https://www.youtube.com/watch?v=fkcIH6DeBUM
[youtube] fkcIH6DeBUM: Downloading webpage
[youtube] fkcIH6DeBUM: Downloading video info webpage
[download] Destination: Exclusive Gabe Newell Interview at Valve HQ subtitled-fkcIH6DeBUM.f137.mp4
[download] 100% of 264.38MiB in 02:27
[download] Destination: Exclusive Gabe Newell Interview at Valve HQ subtitled-fkcIH6DeBUM.f140.m4a
[download] 100% of 14.89MiB in 00:09
[ffmpeg] Merging formats into "Exclusive Gabe Newell Interview at Valve HQ subtitled-fkcIH6DeBUM.mp4"
ERROR: Conversion failed!