Calculation in the Narrows

Start:
   P0 P1 P2 P3 P4
Next:
      P1 P2 P3 P4
      P0
Next (so, P1 and P2 can exchange information as can P0 and P3):
         P2 P3 P4
         P1 P0
Next:
            P3 P4
            P2 P1 P0
Next:
               P4
               P3 P2 P1 P0
End of round
Next:
   P4 P3 P2 P1 P0
Next (now P4 leads):
   P3 P2 P1 P0
   P4
Next:
   P2 P1 P0
   P3 P4
Next:
   P1 P0
   P2 P3 P4
Next:
      P0
      P1 P2 P3 P4
Next:
   P0 P1 P2 P3 P4
end of second round

Figure 1: People moving in a corridor.