N = input()
ans = 0
for t in range(N):
	S1, S2 = raw_input().split()
	H1, M1 = map(int,S1.split(':'))
	H2, M2 = map(int,S2.split(':'))
	tmp = (H2+24)*60 + M2 - (H1*60 + M1)
	if tmp >= 1440:
		tmp -= 1440
	ans += tmp
print ans