n = int(input())
ans = 0
for i in range(n):
    s, t = map(str, input().split())
    h1, m1 = map(int, s.split(':'))
    h2, m2 = map(int, t.split(':'))
    s = h1*60+m1
    t = h2*60+m2
    if s <= t:
        ans += t-s
    else:
        ans += t+24*60-s
print(ans)