from datetime import datetime, timedelta
N=int(input())
ans=0
for _ in range(N):
    a,b=input().split()
    fmt='%H:%M'
    a=datetime.strptime(a,fmt)
    b=datetime.strptime(b,fmt)
    if b<a: b+=timedelta(days=1)
    diff=(b-a).total_seconds()//60
    ans+=int(diff)
print(ans)