from datetime import datetime as dt
n = int(input())
c = []
for i in range(n):
    a, b = map(str, input().split())
    at = dt.strptime(a,"%H:%M")
    bt = dt.strptime(b,"%H:%M")
    c.append((bt-at).seconds) #足し引きすると timedelta
print(sum(c)//60)