Annuity: Plan Cost at Starting Date

I have a federal pension.  Turbo Tax this year is asking for the annuity plan cost at the starting date (1995);  In past years, Turbo Tax calculated the difference between the full annuity and taxable amount by carrying over from previous years -- about $1,400 each year.   Turbo Tax uses the simplified method.  How do I find the original cost at the starting date?