import strutils, sequtils
proc proc_A(h, m: int): auto = h * 60 + m

let N = stdin.readline.parseInt
var ttl = 0

for j in 0 ..< N:
  let HMHM = stdin.readLine.split
  var slp_T, awk_T = 0
  for i, HM in HMHM:
    let t = HM.split(':').map parseInt
    let (H, M) = (t[0], t[1])

    if i == 0: slp_T = proc_A(H, M)
    if i == 1: awk_T = proc_A(H, M)

  ttl += awk_T - slp_T + 24 * 60 * int(slp_T > awk_T)

echo ttl