N = int(input())
T = [0 for _ in range(24 * 60 + 1)]

res = 0
for i in range(N):
    s, t = map(lambda s: list(map(int, s.split(":"))), input().split())
    s = s[0] * 60 + s[1]
    t = t[0] * 60 + t[1]
    if s > t:
        t += 24 * 60
    res += t - s

print(res)