import sequtils, strutils

let
  n = parseInt readLine stdin
  xs = (0 ..< n).mapIt split readLine stdin

proc fn(s: string): int =
  var h, m: int
  (h, m) = s.split(':').map parseInt
  h * 60 + m

var ans = 0
for x in xs:
  var t = x[1].fn - x[0].fn
  if t < 0: t.inc (24 * 60)
  ans.inc t

echo ans