6. 180 Determine all functions \( f: \mathbb{R} \rightarrow \mathbb{R} \), where \(\mathbb{R}\) is the set of real numbers, such that for all \( x, y \in \mathbb{R} \), the following holds:
\[ f(x - f(y)) = f(f(y)) + x f(y) + f(x) - 1 \]

establish.