N = int(input())
ans = 0
for _ in range(N):
    sleep, awake = map(str,input().split())
    H1,H2 = map(int,sleep.split(":"))
    h1,h2 = map(int,awake.split(":"))

    if H1 > h1 or (H1 == 0 and h1 == 0 and H2 > h2):
        h1 += 24
    ans += (h1-H1)*60 + h2-H2
print(ans)