n = int(input())
res = 0
for _ in range(n):
    p,q = input().split()
    a,b = map(int, p.split(':'))
    c,d = map(int, q.split(':'))
    c += 24
    mn = c * 60 + d - a * 60 - b
    mn %= 24 * 60
    res += mn
print(res)