Three processes compose a thermodynamic cycle shown in the accompanying P-V, diagram of an ideal gas.

Process 1$\to$2 takes place at constant temperature, during this process 60 J of heat enters the system. Process 2$\to$3 takes place at constant volume. During this process 40 J of heat leaves the system. Process 3$\to$1 is adiabatic. What is the change in internal energy of the system during process 3$\to$1?