Here is my code :

Const    Eps = 0.01 ;

Var     AC , AD , BC , BD , CD , BC2 , BD2 , AC2 , AD2 : Extended ;

BEGIN
     Read( AC , AD , BC , BD ) ;
     BD2 := BD * BD ;
     AC2 := AC * AC ;
     BC2 := BC * BC ;
     AD2 := AD * AD ;
     If Abs( AC * AD - BC * BD ) < Eps then Begin
        Writeln('Impossible') ; Halt ;
     End ;

     CD := (AC2 + AD2) * BC * BD - (BC2 + BD2) * AC * AD ;
     CD := CD / ( BC * BD - AC * AD ) ;

     If CD < 0.00000 then Writeln('Impossible.')
     Else Begin
         CD := Sqrt( CD ) ;
         Writeln('Distance is ',CD*1000:0:0,' km.') ;
     End ;
END.