You can find your admin URI from app/etc/local.xml file,
Just go to local.xml file using your FTP and edit local.xml file,
You can see at the end of file with one section called, admin routers,
<admin>
<routers>
<adminhtml>
<args>
<frontName><![CDATA[admin]]></frontName>
</args>
</adminhtml>
</routers>
</admin>
Now in above section your uri coming from line,
<frontName><![CDATA[admin]]></frontName>
Copy above text after CDATA and use as suffix to your main store.
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