from datetime import datetime
n = int(input())
cnt = 0
s_format = "%H:%M"
for _ in range(n):
    x, y = input().split()
    p = datetime.strptime(x, s_format)
    q = datetime.strptime(y, s_format)
    z = q - p
    cnt += z.seconds

print(int(cnt / 60))