Robot Energy Source Order

You design a robot that explores places people cannot reach, and you want it to travel as far as possible. You have $n$ energy sources. Source $i$ accelerates the robot at $a_i$ $\text{m/s}^2$ and keeps doing so for $s_i$ seconds. The robot starts at rest, so its initial velocity is 0.

Once you pick a source you use it for all of its $s_i$ seconds, then switch straight to a source you have not used yet. The switch takes no time, and each source can be used only once. You choose the order of the sources so that the distance traveled is as large as possible.

Given the acceleration and the duration of every source, write a program that computes the distance traveled under the best order minus the distance traveled under the order given in the input.

Physics background: if the velocity is $v$ right before you start a source whose acceleration is $a$, then after $t$ seconds the robot has traveled $vt + \frac{1}{2}at^2$ more meters and its velocity is $v' = v + at$.

The first line contains the number of energy sources $n$ ($1 \le n \le 10^4$). Each of the next $n$ lines contains the acceleration $a_i$ and the duration $s_i$ of one source, separated by a space. Both values are positive integers no greater than $10^4$.

Print the distance traveled under the best order minus the distance traveled under the input order, with exactly one digit after the decimal point. The difference is always an integer, so that digit is always 0.