def to_min(hhmm: str) -> int:
    h, m = map(int, hhmm.split(':'))
    return h * 60 + m


N = int(input())
ans = 0

for i in range(N):
    b, w = map(to_min, input().split(' '))
    if b > w:
        b -= 24 * 60
    ans += w - b

print(ans)