var n,i,ans,num,cnt0,cnt1:longint;
    a:array[1..500]of longint;
begin
 readln(n);
 for i:=1 to n do read(a[i]);
 for ans:=0 to n do begin
  num:=ans-1;
  cnt0:=0;cnt1:=0;
  for i:=1 to n do if a[i]=num then inc(cnt0);
  for i:=1 to n do if a[i]=ans then inc(cnt1);
  if (cnt0=ans)and(cnt0+cnt1=n)then begin
   writeln(ans); close(input);close(output);halt;
  end;
 end;
  writeln('-1');
close(input);
close(output);
end.