Hi and welcome to KH. Are you knitting the knits and purling the purls? I think that's what you have going. After you do your decrease, look at the next stitch. If it's a knit, purl it and continue on in pattern. If it's a purl, knit it and continue on. A knit stitch will look like a V and purl stitch will have a bar across.
I should never overestimate my basic arithmetic skills.