why the sample output is :
1
4
1 2
1 3
2 3
3 4
?
I think that 3 connections would be enough,right?
e.g.:
1
3
1 2
2 3
3 4
Also in the problem description it is stated that : "There is another problem - not each hub can be connected to any other one because of compatibility problems and building geometry limitations." But it isn't mentioned anywhere further! Does it mean only that not all connections are possible - i mean that if it isn't stated - there is connection from a to b - so we can't use it? plese help!

It does not want MST or minimum edge num.
It want minimum the longest cable connecting two hubs.
In other words,the weight of the longest edge connecting the vertices is minimal.

And your second question,your idea is right.It means,if it says there's an edge connecting a and b,it can be used,if not,we can't use it.

Good luck!