The three processes in a thermodynamic cycle shown in the figure are : Process $1 \rightarrow 2$ is isothermal; Process $2 \rightarrow 3$ is isochoric (volume remains constant); Process $3 \rightarrow 1$ is adiabatic. The total work done by the ideal gas in this cycle is $10 \mathrm{~J}$. The internal energy decreases by $20 \mathrm{~J}$ in the isochoric process. The work done by the gas in the adiabatic process is $-20 \mathrm{~J}$. The heat added to the system in the isothermal process is