Sum column every n column in a data frame R
<p>I have a <code>df(A)</code> with 10 column and 300 row. I need to sum every two column, between them, in this way: </p>

<pre><code>A[,1]+A[,2] = # first result
A[,3]+A[,4] = # second result
A[,5]+A[,6]= # third result
....
A[,9]+A[,10] # last result
</code></pre>

<p>The expected final result is a new dataframe with 5 column and 300 row.
Any way to do this? with <code>tapply</code> or loop <code>for</code>? 
I know that i can try with the upon example, but i'm looking for a fast method
Thank you</p>