Why is the amortization on net gain (loss) 0?

Brief Exercise 17-10 (Algo) Pension expense; prior service cost [LO17-6] The pension plan was amended last year, creating a prior service cost of $140 million. Service cost and interest cost for the year were $38 million and $18 million, respectively. At the end of the year, there was a negligible balance in the net gain-pensions account. The actual return on plan assets was $18 million although it was expected to be $20 million. On average, employees' remaining service life with the company is 20 years. What was the total pension cost for the year? (Amounts to be deducted should be indicated with a minus sign.)