First sorry for my English.
May I know how to create a conditional function for this reason.

I create a function in A1 cell using data validation function list option.
After that I can select in A1 cell as 2000, 2001, 2002, 2003 like that.

I typed in I column as I3=1 , I4=2 , I5=3

Now I want to prepare a condition in A3 cell If A3=2000 A3 = I3 and
If A3=2001 A3 = I4 and If A3=2002 A3 = I4 and If A3=2003 A3 = I5 like that.

Please help me to create this condition.