In the diagram, the graph between volume and pressure for a thermodynamic process is shown. If $U_{\mathrm{A}}=0$,$U_{\mathrm{B}}=20\mathrm{ }\mathrm{J}$ and the energy given from $\mathrm{B}$ to $\mathrm{C}$ is $30\mathrm{ }\mathrm{J}$, then at the stage of $\mathrm{C}$, the internal energy of the system is