{$N+}
Program Mars_satellites;

 Var
   cos_,x,a,b,c,d:Double;

Begin
  Readln(a,b,c,d);
  If (a*b-c*d)=0 Then
  Begin
    Write('Impossible');
  End
  Else
  Begin
    cos_:=a*a+b*b-c*c-d*d;
    cos_:=cos_/(2*a*b-2*c*d);
    x:=a*a+b*b-2*a*b*cos_;
    x:=sqrt(x);
    x:=round(1000*x);
    Write('Distance is ',x:0:0,' km.');
  End;
End.

Hi,
your program has two mistakes, first one is (a*b-c*d)=0, equlity cannot be tested with doubles that way, you should use Abs(a*b-c*d)<e instead, where e is very small value, in this case e=0.01 is enough; second mistake is the formula I think, try to solve the math part of this problem again. If you want I can send you my AC program ( or just it's formula ).

...то выведите "Impossible.", иначе...
"Impossible{.}"

who said the numbers were double? they are small integers...
And the formula is very good...

working formula =) nice