def parse(a, b) :
    ha, ma = map(int, a.split(":"))
    hb, mb = map(int, b.split(":"))
    if ha >= hb and ma > mb :
        ha -= 24
    return (hb-ha)*60 + (mb-ma)


c = 0
N = int(input())
for i in range(N) :
    a,b = input().split()
    c += parse(a,b)
print(c)