Connecting Windows AD with Office 365 for small/medium company

Hi i have a customer with Active Directory Windows 2012 r2 Server in office. Now they've bought 10 seat of Office 365, so i would connect AD with Office 365 in order to synch users and password with SSO.
i need to know exactly what i have to do; this customer has no many resources (ram, windows license, firewall powerful..) to create the entire ADFS object.
Is there a simple but secure way ? If i not realize this connection, i have two different system with no kind of synchronization, but they works?
Really thanks, ask me for details!
Mattia MinerviniAsked:
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Vasil Michev (MVP)Commented:
AD FS is not needed for this scenario, take a look at AAD Connect and the pass-trough authentication feature:

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