title: Error reading dataset in R
body: <p>I have problem in reading a dataset </p>
  
  <p>My code : </p>
  
  <pre><code>require(igraph)
  g &lt;- graph(c(0, 1, 1, 2, 2, 0, 1, 3, 3, 4, 
                 4, 5, 5, 3, 4, 6, 6, 7, 7, 8, 
                 8, 6, 9, 10, 10, 11, 11, 9))
  </code></pre>
  
  <p>Error :</p>
  
  <pre><code>Error in graph(c(0, 1, 1, 2, 2, 0, 1, 3, 3, 4, 4, 5, 5, 3, 4, 6, 6, 7,  : 
    At structure_generators.c:84 : Invalid (negative) vertex id, Invalid vertex id
  </code></pre>