N = int(input())

t = 0
for _ in range(N):
    a, b = input().split()
    hs, ms = map(int, a.split(':'))
    he, me = map(int, b.split(':'))
    s = hs*60 + ms
    e = he*60 + me
    
    t += e - s + [0, 1440][ s > e]

print(t)