The page-cities table is the coupled data table? Or is it a fully custom table you created?
If it is fully custom table, then you can save whatever value you want or keep it as identifier column with auto-increment and the SQL server will take care.
If it is the coupled data table of given page type, you should leave the value to be set by Kentico. The ID column in the coupled data table has to match the DocumentForeignKeyValue column in the CMS_Document table.