N = input()

ans = 0
for n in range(N):
    sleep, getup = raw_input().split()
    sleep = sleep.split(":")
    getup = getup.split(":")
    hour = (int(getup[0]) - int(sleep[0])) % 24
    min = (int(getup[1]) - int(sleep[1])) 
    ans += 60 * hour + min

print ans