Program Captain;
Var n,k,i,j,t,max:longint;
a:array[0..100000]of longint;
F:array[0..2000001]of longint;
Begin
Assign(input,'xorxor.in');Reset(input);
Assign(output,'xorxor.out');Rewrite(output);
	Readln(n,k);t:=0;max:=-1;
	For i:=1 to n do Read(a[i]);
	For i:=1 to n-1 do
		For j:=i+1 to n do begin 
			t:=a[i] xor a[j];
			inc(F[t]);
			If t>max then max:=t;
		End;i:=-1;
	While k>0 do begin
		inc(i);
		If f[i]>0 then begin 
			DEC(K);Dec(f[i]);
			If f[i]>0 then Dec(i);
		End;
	End;
	Writeln(i);
Close(input);Close(output);
End.