The inventory for a part is replaced every 4 hours. Demand for the part is at the rate of 0.5 parts per hour. How much inventory should be kept for a 99% service level?

The inventory for a part is replaced every 4 hours. Demand for the part is at the rate of 0.5 parts per hour. How much inventory should be kept for a 99% service level?

Assume that demand is Poisson distributed.