title: Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_concat_0
body: <p>I first converted 2 gifs of same width and height to mp4 using ffmpeg options</p>
  <pre><code>ffmpeg -i video1.gif -movflags faststart -pix_fmt yuv420p -vf &quot;scale=trunc(iw/2)*2:trunc(ih/2)*2&quot; video1.mp4
  ffmpeg -i video2.gif -movflags faststart -pix_fmt yuv420p -vf &quot;scale=trunc(iw/2)*2:trunc(ih/2)*2&quot; video2.mp4
  </code></pre>
  <p>afterwards I joined with</p>
  <pre><code>ffmpeg -i video1.mp4 -i video2.mp4 -filter_complex &quot;concat=n=2:v=1:a=1&quot; -f MP4 -y video.mp4
  </code></pre>
  <p>But I got error</p>
  <pre><code>Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_concat_0
  </code></pre>
  <p>with other mp4 in the past joining works so I don't understand why.</p>