Upper panel: masses in various phases transformed from cold molecular gas around an expanding HII region at the termination time as a function of the mass of a central star. The red dot-dashed line, the blue dotted line, and purple dashed line correspond to ionized hydrogen in the HII region, neutral hydrogen in the photodissociation region, and warm molecular hydrogen gas without CO, respectively. The uppermost black solid line denotes the total mass of these non-star-forming gases. Lower panel: the IMF-weighted mass of non-star-forming gas transformed from molecular gas by a massive star of mass M_∗. The area under the curve is proportional to the mass generated by massive stars whose mass function follows dn/d, −β + 1 = −1.3 (blue dashed), −1.5 (black solid), −1.7 (red dotted). The peak of the curve determines the inverse of star formation efficiency ϵ_SF (see explanation below Eq. (7)).