Tasks and deadlines

You have $n$ tasks; task $i$ takes $a_i$ time and has deadline $d_i$. You do the tasks one after another without breaks, starting at time $0$. If a task is finished at time $f$, you get $d_i - f$ points (possibly negative). What is the maximum total number of points?

The first line contains $n$ ($1 \le n \le 2 \cdot 10^5$). The next $n$ lines contain $a_i$ and $d_i$ ($1 \le a_i, d_i \le 10^6$).

A single number — the maximum total points.

The total is $\sum d_i - \sum f_i$. The first part does not depend on the order, so the sum of finishing times must be minimised: do the tasks by increasing duration. If a short task follows a long one, swapping them decreases the sum. Deadlines affect the answer but not the order.

In the second example the order is $2, 3, 4, 4$ and the answer is $5 + 0 - 7 - 8 = -10$. The answer reaches $-2 \cdot 10^{16}$ — a 64-bit type is needed.

Source: CSES, problem 1630.