Sam purchased a house in San Francisco, CA, in 1952 for $15,000. He never married but had several children over the years, none of whom he has ever acknowledged. Sam has multiple grandchildren, one of which is named Sam, but he knows none of them. In 2020, Sam sold his house, which was his primary residence since he purchased it, for $5,000,000. He then moved into a retirement community for single octogenarians. How much is Sam's taxable gain from the sale of his house?