Public debt is the sum of deficits and surpluses (negative deficits) over time. Suppose that a country has no public debt in year 1 but experiences a budget deficit of $40 billion in year 1, a budget deficit of $20 billion in year 2, a budget surplus of $10 billion in year 3, and a budget deficit of $2 billion in year. What is the absolute size of its public debt in year 4?