Scanf.scanf "%d" (fun n ->
    let rec loop i acc =
        if i = n then acc else
            let acc = acc + Scanf.scanf " %d:%d %d:%d" (fun h1 m1 h2 m2 ->
                (h2 * 60 + m2 - (h1 * 60 + m1) + 1440) mod 1440
            )
            in
            loop (i + 1) acc
    in
    loop 0 0 |> Printf.printf "%d\n"
)