The melody of 18 by 5 Seconds of Summer runs 383 notes across 71 bars, 2:56 long in D at 160 beats per minute. The tab sits almost entirely in the first two frets, one note at a time — no chords to barre and no strumming pattern to keep up with. A piano backing track plays the chords and bass underneath, so the line you are learning sounds like the record even at half speed.
Prefer to strum and sing it instead? 18 chords and lyrics.