import datetime
N = input()
res = 0
for i in range(N):
    s,e = raw_input().split()
    st = datetime.datetime.strptime(s, "%H:%M")
    et = datetime.datetime.strptime(e, "%H:%M")
    if (et < st):
        st -= datetime.timedelta(1)
        delta = et - st
    else:
        delta = et -st
    res += delta.total_seconds()
print int(res / 60)