Simplify the following text for a Grade 12 native English-speaking person.
Original: At the Congress of Vienna of 1814 â `` 15 , the territory of Geneva was extended to cover 15 Savoyard and six French parishes , with more than 16,000 Catholics ; at the same time it was admitted to the Swiss Confederation .
Simplified: At the Congress of Vienna ( 1814-15 ) the territory of Geneva was enlarged to cover 15 Savoyard and 6 French parishes , with more than 16,000 Catholics ; at the same time it became a part of the Swiss Confederation .