title: match records of a file with field values of another file in awk
body: <p>I want to sum up values in a field of a CSV file
  where matching should be checked by reading another file,
  say we have CSV_file:</p>
  
  <pre><code>adam,18
  denis,19
  julie,17
  adam,15
  max,20
  julie,19
  </code></pre>
  
  <p>and a simple txt file containing:</p>
  
  <pre><code>adam
  julie
  </code></pre>
  
  <p>all I need is to sum up 18,15,17,19
  how could I easily do that with awk?</p>