n = int(input())

ans = 0
for i in range(n):
    [h1, m1], [h2, m2] = [map(int, x.split(':')) for x in input().split()]
    t1 = h1 * 60 + m1
    t2 = h2 * 60 + m2 
    if t2 <= t1:
        t2 += 60 * 24
    ans += t2 - t1
print(ans)