n = int(raw_input())
s = 0
for _ in range(n):
    t1,t2 = raw_input().split()
    h1,m1 = map(int, t1.split(':'))
    h2,m2 = map(int, t2.split(':'))
    d = (h2*60+m2) - (h1*60+m1)
    if d < 0:
        d += 1440
    s += d
print s