USA the minor's act

[Tish]

Two years after my father died in 2013, I found a document that indicated that 500 shares of a publicly owned company. Those shares were in my name but he was the custodian under the 'minor's act.' I was then 61 at at the time. The shares were transferred then and I am now about to sell them. How is a cost basis determined? Would it be the price on the day/date when I turned 18 or on the date of my dad's death? Please help! Thank you.