n=int(input());ans=0
for i in range(n):
	a,b=input().split()
	q,w=a.split(":")
	e,r=b.split(":")
	a=int(q)*60+int(w)
	b=int(e)*60+int(r)
	if a>b:
		b+=24*60
	ans+=b-a
print(ans)