A study of the sales lost when a store has insufficient inventory to meet the demand and impatient customers are unwilling to wait for the store to reorder has found that when the demand is a decreasing exponential function, the expected sales lost per period is given by

where B is the base stock level and k is a positive constant. Find the value of the expected sales lost.

E = [(y - B). B kBe ky 1 - e-kB e-kB dy,