step by step solution needed, urgency

Production planning: Direct Co sells an item whose demand over the next 4 months is 100,140,210, and 180 units, respectively. The company can stock just enough supply to meet each month's demand, or it can overstock to meet the demand for two or more successive and consecutive months. In the latter case, a holding cost of $1.20 is charged per overstock unit per month. Direct Co estimated the unit purchase prices for the next 4 months to be $15, $12, $10, and $14, respectively. A setup cost $200 is incurred each time a purchase order is placed. The company wants to develop a purchasing plan that will minimize the total costs of ordering, purchasing, and holding the item in stock. Formulate the problem as a shortest-route model