let () =
  let n = read_int () in
  let rec doit i acc =
    if i = n then acc
    else
      let a, b, c, d = Scanf.scanf "%d:%d %d:%d " (fun a b c d -> a, b, c, d) in
      let c = if a = c && b > d || a > c then c + 24 else c in
      doit
        (i + 1)
        (acc + (c - a - if b > d then 1 else 0)*60 + d - b + if d < b then 60 else 0) in
  doit 0 0 |> Printf.printf "%d\n"