uses crt;
var n,k,i,j,dem,tg:longint;
    kt:boolean;
    a,b:array[0..100000] of int64;
PROCEDURE NHAP;
BEGIN
 readln(n,k);
 KT:=TRUE;
 IF (N<=0) OR (N>10000) OR (K<=0) OR (K>N) THEN BEGIN KT:=FALSE; EXIT; END;
  for i:=1 to n do
  begin
  read(a[i]);
      IF (A[I]<=0) OR (A[I]>1000000000000) THEN BEGIN KT:=FALSE; EXIT; END;
  end;
end;
procedure xl;
begin
 for i:=1 to n do
 for j:=i+1 to n do
   if a[i]<a[j] then
   begin
   tg:=a[i];
   a[i]:=a[j];
   a[j]:=tg;
   end;
   dem:=a[k] div 2+1;
   writeln(dem);
 end;
 begin
 clrscr;
 nhap;
 IF KT=TRUE THEN xl ELSE WRITELN('-1');
readln;
readln;
 end.