C# - How to add a reference do system.numerics.dll
<p>I want to use the BigInteger class from the System.Numerics but if i want to write </p>

<pre><code>using System.Numerics
</code></pre>

<p>Numerics is not found.
I searched the web and I found that I have to add a reference to System.Numerics.dll
But how can I do that?</p>
