from datetime import datetime, timedelta


def main():
    n = int(input())
    t = timedelta(0)
    for _ in range(n):
        a, b = map(lambda x: datetime.strptime(x, "%H:%M"), input().split())
        if a > b:
            b += timedelta(days=1)
        t += b - a

    print(int(t.total_seconds()) // 60)
    
if __name__ == '__main__':
    main()