In an adiabatic irreversible compression process, suppose the pressure P(ext) compresses the piston and does some work say 50J.(50J of energy is lost by surrounding). Since the pressure P(int) is less than the P(ext), the work done by system on surrounding will be -20J(suppose). Negative sign shows that 20J of energy is lost my surrounding. But initially we saw that surrounding lost 50J! How to tackle this discrepancy? I am very confused.