# An Easy IMO Problem!

$$d$$ is a positive integer not equal to $$2, 5$$ or $$13$$. How many values of $$d$$ are there such that for any pair $$(a, b)$$ from the set $$\{2, 5, 13, d\}$$, $$ab-1$$ is a perfect square?

###### This problem is a part of the set "Olympiads and Contests Around the World - 2". You can see rest of the problems here.
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