The Oxford Olympics Study 2016 found that sports-related costs for the Summer Games since 1960 are on average USD 5.2 billion and for the Winter Games USD 3.1 billion. This does not include wider infrastructure costs like roads, urban rail, and airports, which often cost as much or more than the sports-related costs. The most expensive Summer Games are Beijing at USD 40-44 billion and the most expensive Winter Games are Sochi 2014 at USD 51 billion. As of 2016, costs per athlete are on average USD 599,000 for the Summer Games and USD 1.3 million for the Winter Games. For London 2012, cost per athlete was USD 1.4 million; for Sochi 2014, USD 7.9 million.
Question: How many billions more expensive was the Sochi 2014 Winter games than the Beijing Summer Games at their highest estimate?
Answer: 7