title: how to detect a string of records in a input.file with no costant NF?
body: <p>I've a input_file.txt in this way:</p>
  
  <pre><code>1
  0.421934 0.421622 384000
  0.419202 0.418349 384000
  0.422398 0.422148 384000
  0.422691 0.422789 384000 D
  0.427295 0.426805 384000
  0.424004 0.423169 384000
  0.428237 0.428279 384000
  2
  0.422842 0.422462 576000
  0.420401 0.419927 576000
  0.42179 0.421462 576000
  0.424359 0.424637 576000 F
  0.425756 0.425569 576000
  0.421996 0.421274 576000
  0.428395 0.428422 576000
  </code></pre>
  
  <p>I'd like to have an output_file.txt as:</p>
  
  <pre><code>1 0.422691 0.422789 384000 D
  2 0.424359 0.424637 576000 F
  </code></pre>
  
  <p>(I put the letters D and F only to indicate easily the two lines that I want)</p>
  
  <p>How can I do this using awk?</p>
  
  <p>Thank you</p>