title: AWK, Arithemetical OP on odd line
body: <p>Good day,</p>
  
  <p>I was wondering how to do an arithmetical op with awk, only on odd lines.</p>
  
  <p>So far I have done: </p>
  
  <pre><code>SUM=100 ; awk -v var1=$SUM '{sum = $1 + var1 ;print $(NR%2=1?sum:"0")}' file
  </code></pre>
  
  <p><strong>INPUT</strong></p>
  
  <pre><code>1
  2
  3
  4
  </code></pre>
  
  <p><strong>Expected output</strong></p>
  
  <pre><code>101
  2
  103
  4
  </code></pre>
  
  <p>Thank in advance for any clue or suggestion.</p>