on august 15, blockberry published the official tracklist for "[+ +]" (read as "plus plus") as their debut extended play as a full group. the extended play included "favorite" and was released on august 20, 2018 alongside the title track, "hi high". it started at number 2 on south korea's gaon album chart, and was the second best-selling debut album by a girl group in 2018. "[+ +]" was repackaged as "[x x]" (read as "multiple multiple") on february 19, 2019, with six more tracks, including a new single, "butterfly". on august 17, 2019, loona made their first appearance in the united states, performing at kcon 2019 in los angeles. on december 13, 2019, loona released a single called "365" as an appreciation song for their fans.