Average nucleon-nucleon impact parameter as a function of the number of participants for p-Pb at $\snn$ = 5.02 TeV from a Glauber MC calculation as implemented in HIJING (no shadowing, no elastic scattering). The result depends on the modelling of the spatial parton density in the nucleon. In HIJING it is approximated by the Fourier transform of a dipole form factor.