In the Xray lab at KTH some experiments require
evacuated tubes between source and sample and between sample
and detector so that the Xrays are not absorbed by the air.
Since the positions of object and detector vary between
different experiments, several tubes of different lengths are
available. The tubes should be fixed together in pairs, since
they have a vacuum window only in one end. Two such tube pairs
should be chosen, one to place between the source and the
object and one to place between the object and the detector.
This, however, gives a large set of possible lengths and makes
it difficult to figure out which tubes to use for an
experiment. The length of the tubes used should be as long as
possible to minimize air absorption, but there is a limited
amount of space between source and object
$L_1$ and between object and detector
$L_2$. What is the maximum
length of air that can be replaced by vacuum tubes in this way?
Task
Given a set of tube lengths and the two distances
$L_1$ and $L_2$, find four tubes with the total
length being as long as possible under the constraint that the
sum of the first two tube lengths is at most $L_1$ and the sum of the last two tube
lengths is at most $L_2$.
Input
The first line of input contains three positive integers,
$L_1$ and $L_2$, denoting the distances
explained above in mm ($1 \leq
L_1, L_2 \leq 10\, 000$) and $N$, the number of available tubes
($4 \leq N \leq 2\, 000$).
The following $N$ lines
each contain a single positive integer less than or equal to
$10\, 000$, the length of
a tube in mm.
Output
Output one line containing the maximum total length of air
that can be avoided, i.e., the sum of the four tubes chosen. If
there are no two pairs of tubes fitting into the setup, output
the single word “Impossible” instead.
Sample Input 1 
Sample Output 1 
1000 2000 7
100
480
500
550
1000
1400
1500

2930

Sample Input 2 
Sample Output 2 
200 300 6
100
100
200
200
300
300

Impossible
