In a surface mine bench, overburden is removed by the shovel-dumper combination. For the dumper:

Time required at the loading station : 3.0 min

Time required at the unloading station : 1.0 min

Distance between loading and unloading stations : 4.5 km

Average speed during loaded travel : 12.0 km/hr

Average speed during empty travel : 18.0 km/hr

Minimum number of dumpers required to avoid idle time of the shovel, is ______. (in integer)