let () =
	let n = read_int () in
    let arr = Array.init n @@ fun _ -> Scanf.scanf "%d " @@ fun d -> d in
    let ans = Array.fold_left (fun (x, i) a -> if a = x then (x+1, i) else (x, i+1)) 1 arr in
    print_int (if (snd ans) = Array.length arr then -1 else snd ans)