n = int(input())

total_sleep = 0
for i in range(n):
    time = input().split()
    sleep_h = int(time[0].split(":")[0])
    sleep_m = int(time[0].split(":")[1])
    wake_up_h = int(time[1].split(":")[0])
    wake_up_m = int(time[1].split(":")[1])
    if sleep_h > wake_up_h:
        wake_up_h += 24
    total_sleep += wake_up_h * 60 + wake_up_m - sleep_h * 60 - sleep_m
print(total_sleep)