To get an even sum, either both would have to select even numbers, or both would have to select odd numbers. Since these events are disjoint, you can use the relationship:
P(sum is even) = P(George chooses and even number and Wally chooses an even number) + P(George chooses an odd number and Wally chooses an odd number)
Can you take it from here?
If not, try a jar with 4 balls numbered 1- 4 and list all the possible outcomes.