Ben,  Billy,  Bob and Blake have gems.
 
Ben - has a round, blue, sapphire;
Billy - has an oval, blue, sapphire;
Bob - has a round, white, diamond;
Blake - has an oval, white, diamond.
 
They placed the ads about selling of the gems in the Internet.
 
Ben: A beautiful blue sapphire is for sale;
Billy: An excellent blue sapphire is for sale;
Bob: An excellent white diamond is for sale;
Blake: A beautiful white diamond is for sale.
 
Bonnie wants to buy round gems.
How can she get only Ben's and Bob's ads in Internet?

Additional condition:


They don't know each other.
Only Sergey Brin knows the exact solution.
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