  The novel method is proposed for the reconstruction of the early bolometric
light curve for supernovae IIP based on a set of spectra of H alpha emission
during the first day after the shock breakout. The method exploits an effect of
the radiative acceleration of the preshock circumstellar gas that is manifested
in the broad H alpha wings. The efficiency of the method is demonstrated in the
case of SN 2013fs with spectra taken between 6 and 10 hours after the shock
breakout. The exceptional feature of this method is that it does not require
the photometry, the distance, and the extinction.
