Array Formula in A15:

=SUM(IF(COUNTIF(List1, List1)+COUNTIF(List2, List1)=1, List1)+IF(COUNTIF(List2, List2)+COUNTIF(List1, List2)=1, List2)) + CTRL + SHIFT + ENTER

Named ranges
List1 (A2:A11)
List2 (B2:B11)
What is named ranges?

Download excel sample file for this tutorial.
Comparing two columns and sum unique values.xls
(Excel 97-2003 Workbook *.xls)

Functions in this article:

COUNTIF(range,criteria)
Counts the number of cells within a range that meet the given condition

SUM(number1,[number2],)
Adds all the numbers in a range of cells

IF(logical_test;[value_if:true];[value_if_false])
Checks whether a condition is met, and returns one value if TRUE, and another value if FALSE

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