from re import split
n = int(input())
minute = 0

for _ in range(n):
    t1, t2 = input().split()
    h1, m1 = map(int, t1.split(":"))
    h2, m2 = map(int, t2.split(":"))

    tt1 = h1 * 60 + m1
    tt2 = h2 * 60 + m2

    minute += tt2 - tt1 if tt2 - tt1 > 0 else (tt2 + 24 * 60) - tt1

print(minute)