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1294. Mars Satellites

Time limit: 1.0 second
Memory limit: 64 MB
Four artificial satellites travel in one plane along the areostationary orbit around Mars. They have code names A, B, C and D and travel exactly in this order. Venus’s scouts for military purposes (for what particular purpose they did not say) decided to find a distance between satellites C and D. All Mars satellites could measure distances to the other satellites, that is why all what is needed to do is to penetrate in the computer system of satellite C and measure the distance to satellite D (or vice versa). Nevertheless, Martians are not so stupid and have not very bad defense. That is why all what could Venus’s scouts do is to break the defense of satellites A and B (that were older models). They measured distances from satellites A and B to satellites C and D, but now they do not know how to find the distance from C to D using these measurements. You can help them.

Input

There are 4 numbers: distances from A to D, from A to C, from B to D and from B to C in thousands kilometers (integers from 1 to 10000). Satellites can measure distance even through the planet and you may assume that orbit is a circle. Do not assume the radius of the orbit equal to 20392 km as it should be for the real areostationary orbit.

Output

If it is impossible to find out the distance from C to D with these data, you should print "Impossible.", otherwise you are to print "Distance is X km.", where X is the required distance in kilometers (rounded to the integer number).

Sample

inputoutput
4 7 5 7
Distance is 5385 km.
Problem Author: Vladimir Yakovlev
Problem Source: IX Open Collegiate Programming Contest of the High School Pupils (13.03.2004)