n = int(input())
ans = 0
for i in range(n):
    x,y = input().split()
    a,b = x.split(":")
    c,d = y.split(":")
    a = 60*int(a)
    c = 60*int(c)
    a += int(b)
    c += int(d)
    if a > c:
        c += 1440
    ans += c-a
print(ans)