title: How I can stack this data in R
body: <p>I have 15 columns like this:</p>
  <pre><code>v1  v2  v3  v4  v5  v6  v7  v8  v9  v10 v11 v12 v13 v14 v15
  a   b   c   d   e   f   g   h   i   j   k   l   m   n   p
  </code></pre>
  <p>I want to stack it as follows:
  v1 and v2; v3,v4 and v5;v6,v7,v8, v9 and v10; v11,v12 andv13 , v14 and v15</p>
  <p>The outcome is</p>
  <pre><code>V1  V2  V3  V4  V5
  a   b           
  c   d   e       
  f   g   h   i   J
  k   l   m       
  n   p
  </code></pre>