The sovereign state of Austria is a federal republic with a parliamentary representative democracy comprising nine federated states. The capital and largest city, with a population exceeding 1.8 million, is Vienna. Other major urban areas of Austria include Graz, Linz, Salzburg and Innsbruck. Austria is consistently ranked as one of the richest countries in the world by per capita GDP terms. The country has developed a high standard of living and in 2018 was ranked 20th in the world for its Human Development Index. The republic declared its perpetual neutrality in foreign political affairs in 1955. Austria has been a member of the United Nations since 1955, joined the European Union in 1995, and is a founder of the OECD. Austria also signed the Schengen Agreement in 1995, and adopted the euro currency in 1999.