The potential of a point B (-20 m, 30 m) taking the potential of a point A (30 m, -20 m) to be zero in an electric field $\overrightarrow{\mathrm{E}}=10\mathrm{x}\widehat{\mathrm{i}}-20\widehat{\mathrm{j}}{\mathrm{NC}}^{-1}$is