Consider the stress-strain curve for an ideal elastic-plastic strain hardening metal as shown in the figure. The metal was loaded in uniaxial tension starting from $O$. Upon loading, the stress-strain curve passes through initial yield point at $P$, and then strain hardens to point $Q$, where the loading was stopped. From point $Q$, the specimen was unloaded to point $R$, where the stress is zero. If the same specimen is reloaded in tension from point $R$, the value of stress at which the material yields again is _______ MPa.