1. The method of decoding a video signal with a decoding apparatus, comprising: determining, based on a prediction mode relating to a neighboring block, whether at least one of neighboring samples included in the neighboring block is an unavailable sample for an intra prediction, the prediction mode indicating whether the neighboring block is coded in an inter-prediction or the intra-prediction; when at least one of the neighboring samples is determined to be the unavailable sample, substituting an available sample for the unavailable sample, the available sample being adjacent to the unavailable sample; deriving a prediction sample of the current block based on the neighboring samples; and deriving a reconstruction sample of the current block using the prediction sample and a residual sample relating to the current block, wherein when the unavailable sample corresponds to at least one of left neighboring samples other than a top-left neighboring sample, the substitution of the unavailable sample is performed using only a bottom neighboring sample of the unavailable sample, without referring to a top neighboring sample of the unavailable sample, and wherein when the unavailable sample corresponds to at least one of top neighboring samples, the substitution of the unavailable sample is performed using a left neighboring sample of the unavailable sample.