You want to (K1, YO, K1) all into the next stitch. You'll k1 but without slipping the stitch from the left needle, yarn over and k1 again into the same stitch. Now you have 3 loops over the right needle and you can drop the stitch from the left needle. That's a make 3 (M3) out of one stitch or an increase of 2sts. It'll use one of your 13sts and you'll have 12 left to purl.