Results on the primary CR spectrum and composition reconstructed with the SPHERE-2 detector

First preliminary results of the balloon-borne experiment SPHERE-2 on the all-nuclei primary cosmic rays (PCR) spectrum and primary composition are presented. The primary spectrum in the energy range $10^{16}$--$5\cdot10^{17}$ eV was reconstructed using characteristics of Vavilov-Cherenkov radiation of extensive air showers (EAS), reflected from a snow surface. Several sources of systematic uncertainties of the spectrum were analysed. A method for separation of the primary nuclei' groups based on the lateral distribution function' (LDF) steepness parameter is presented. Preliminary estimate of the mean light nuclei' fraction $f_{30-150}$ at energies $3\cdot10^{16}$--$1.5\cdot10^{17}$ eV was performed and yielded $f_{30-150}$= (21$\pm$11)%.