n = int(input())

ans = 0
for i in range(n):
    s, e = input().split()
    hs, ms = map(int, s.split(':'))
    ms += hs * 60
    he, me = map(int, e.split(':'))
    me += he * 60
    if me <= ms:
        me += 60 * 24
    ans += me - ms
print(ans)