n = int(input())
ans = 0

for i in range(n):
    a, b = input().split()
    h, m = map(int, a.split(":"))
    x1 = h * 60 + m
    h, m = map(int, b.split(":"))
    x2 = h * 60 + m
    if x1 > x2:
        x2 += 24 * 60
    ans += x2 - x1

print(ans)