Let's restate the objective once more: the inventory in each product category during any month is equal to the difference between the sum of all orders that we have made up to that month and the sum of all sales that we have made up to that month. In the code cell below, write an expression to compute the monthly inventory based on the difference between the order and sales data, and assign the result to the variable inventory. Hint: You'll want to separately extract the orders and the sales portion of the sales and orders data frame, and use the consume method on each resulting data frame to  get the cumulative totals at each month. Find the difference between the two.