Hi, I don't know what the definition of Corridor amount is

and what is the answer for that? all answers so far are right other than Corridor amount,

The projected benefit obligation and plan assets were $220 million and $300 million, respectively, at the beginning of the year. Due primarily to favorable stock market performance in recent years, there also was a net gain of $80 million. On average, employees' remaining service life with the company is 10 years As a result of the net gain, what was the increase or decrease in pension expense for the year? (Amounts to be deducted should be indicated with a minus sign.) in millions) Net gain 80 Corridor amount 80 Excess, if any 10 Service period (years) 8.00