title: how to fix Non-monotonous DTS in output stream 0:1; when using ffmpeg
body: <p>When I use <code>ffmpeg</code> to convert <code>m3u8</code> to <code>mp4</code>, I get some warning, </p>
  
  <pre class="lang-none prettyprint-override"><code>ffmpeg -i xx.m3u8 -c copy demo.mp4
  </code></pre>
  
  <p>warning is </p>
  
  <pre class="lang-none prettyprint-override"><code>Non-monotonous DTS in output stream 0:1; previous: 3277744, current: 3276712; changing to 3277745. This may result in incorrect timestamps in the output file.
  Non-monotonous DTS in output stream 0:1; previous: 3277745, current: 3277736; changing to 3277746. This may result in incorrect timestamps in the output file.
  </code></pre>
  
  <p>what should I do to fix it?</p>