xml schema help

[nelco]

My xsd:

<xs:element name="UDAS">
<xs:complexType>
<xs:sequence>
<xs:element minOccurs="2" maxOccurs="unbounded" name="UDA">
<xs:complexType>

<xs:sequence>
<xs:element name="UDA_ID" type="inttype" />
<xs:element name="UDA_VALUE" type="inttype" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>

my xml:
<UDAS>
<UDA>
<UDA_ID>32</UDA_ID>
<UDA_VALUE>5</UDA_VALUE>
</UDA>
<UDA>
<UDA_ID>33</UDA_ID>
<UDA_VALUE>47</UDA_VALUE>
</UDA>
<UDA>
<UDA_ID>64</UDA_ID>
<UDA_VALUE>5</UDA_VALUE>
</UDA>
<UDA>
<UDA_ID>63</UDA_ID>
<UDA_VALUE>47</UDA_VALUE>
</UDA>
</UDAS>

I want to make two UDA's mandatory. They are UDA_ID 63 and UDA_ID 64. Others can have UDA_ID's values other than 63 or 64.

How do I restrict that in my schema.

I will appreciate any help.

Thanks