title: FFmpeg audio concat/join not work properly in flutter
body: <p>I am concat/join audio using <a href="https://pub.dev/packages/ffmpeg_kit_flutter" rel="nofollow noreferrer">ffmpeg_kit_flutter</a> with following command. But the joint file duration is not right.</p>
  <pre><code>&quot;-i \&quot;concat:${audioFile1}|${audioFile2}\&quot; -acodec copy $outPath&quot;;
  </code></pre>
  <p>I have two audio files first audio file duration 27s and second audio file duration 1m 47s.</p>
  <p>After join both files actual file duration is 2m 14s.</p>
  <ul>
  <li>But when we input 1st file in first input and 2nd file in second input then the output file duration is 2m 53s.</li>
  <li>And if we input 2nd file in first input and 1st file in second input then the output file duration is 2m 6s.</li>
  </ul>
  <p>Following is my code:</p>
  <pre><code>var cmd = &quot;-i \&quot;concat:${audioFile2.path}|${audioFile1.path}\&quot; -acodec copy $outPath&quot;;
  
  FFmpegKit.executeAsync(cmd, (session) async {
        final returnCode = await session.getReturnCode();
        log(&quot;returnCode $returnCode&quot;);
      });
  </code></pre>
  <p>Please help me how to get the right duration after join both files?</p>