import datetime

n = int(input())

x = 0
for i in range(n):
    t = input().split()
    t0 = datetime.datetime.strptime(t[0], '%H:%M')
    t1 = datetime.datetime.strptime(t[1], '%H:%M')
    t2 = t1 - t0
    h, m = divmod(t2.seconds//60, 60)
    m += h * 60
    x += m
    
print(x)