n=int(input())
ans=0
for i in range(n):
    x,y=input().split()
    a,b=x.split(":")
    c,d=y.split(":")
    if a>c:
        c+=24
    if a==c and b>d:
        c+=24
    ans+=(c*60+d)-(a*60+b)
print(ans)