Scanf.scanf "%d" (fun n ->
    let c = Array.init n (fun _ -> Scanf.scanf " %d %d" (fun a b -> a + b * 4)) in
    Array.sort (fun a b -> compare b a) c;
    let rec loop m i acc =
        if i = n then acc else
            let q = m - c.(i) in
            if q mod 2 = 1 then -1 else
                loop m (i + 1) (acc + q / 2)
    in
    loop c.(0) 1 0 |> Printf.printf "%d\n"
)