fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

fun readTime () =
    let
        val h = readInt ()
        val _ = TextIO.scanStream Char.scan TextIO.stdIn (* skip ":" *)
        val m = readInt ()
    in
        (h, m)
    end


val () =
    let
        val n = readInt ()
        val sleepRecords = List.tabulate (n, fn _ => (readTime (), readTime ()))
                        
        val ans =
            List.foldl
                (fn (((sh, sm), (eh, em)), acc) =>
                    if sh < eh orelse (sh = eh andalso sm <= em)
                    then (eh * 60 + em) - (sh * 60 + sm) + acc
                    else ((eh + 24) * 60 + em) - (sh * 60 + sm) + acc)
                0
                sleepRecords
    in    
        print (Int.toString ans ^ "\n")
    end