Those heels were cute, but they didn't last long! Selena Gomez wore sparkly mid-heel sandals to the premiere of Hotel Transylvania 3: Summer Vacation but she just couldn't keep 'em on her feet -- after the red carpet, Selena ditched the shoes and went barefoot to greet fans and sign autographs outside the theater, on Saturday.

She also added a cashmere hooded sweater over her adorable denim two-piece crop top and skirt by Oscar de la Renta, which frankly kinda ruined the look -- but hey, she was comfortable! Oh, and she topped it all off with $21,500 worth of Tiffany jewels. Still, we'd think twice before going barefoot on an LA sidewalk ... but then she doesn't live here anymore so she probably forgot how nasty the sidewalks are!

Oh and meanwhile ... Justin Bieber's tooling around in his new Lambo ...