It is the most-visited city as measured by international arrivals and has the busiest city airport system as measured by passenger traffic. It is the leading investment destination, hosting more international retailers than any other city. As of 2020, London has the second-highest number of billionaires of any city in Europe, after Moscow. In 2019, London had the highest number of ultra high-net-worth individuals in Europe. London’s universities form the largest concentration of higher education institutes in Europe, and London is home to highly ranked institutions such as Imperial College London in natural and applied sciences, and the London School of Economics in social sciences.In 2012, London became the first city to have hosted three modern Summer Olympic Games.