Summing few columns of a data frame : <p>This is my data</p>

<pre><code>&gt; df&lt;- as.data.frame(matrix(rnorm(15), 3))

&gt; df

          V1         V2         V3          V4         V5
1  0.8757347  1.5984067  1.0295143  0.08161545  1.6208651
2 -0.4039117 -0.9497641 -0.1747716  0.01544082  0.4639266
3 -0.9055205 -0.9686378  0.9451551 -0.05030505 -1.4510613
</code></pre>

<p>How can I add a column at the end that will sum only the last two column? </p>
