I'd go for the 1st then.
The 1st is a bit harder (obviously)... Right now, my son is doing a series of workbook pages that involve picking 2 numbers (out of 5) that add up to one specific number. It's hard for him, but he really likes the way it's done.
I'll see if I have time to pull out the K books tonight and review their contents, but I'd go for 1st. It seems pretty easy.