Cullumber Corporation has a defined-benefit pension plan covering its 1500 employees. Cullumber agrees to amend its pension benefits. The prior service cost associated with the amendment is $750000. The employees are grouped according to expected years of retirement, as follows: Number of Expected Retirement Group Employees on December 31 A 300 2025 B 415 2026 C 540 2027 D 245 2028 What is the overall total number of service years to use for amortization calculations using the years of service approach?