class Time:
    def __init__(self, s: str) -> None:
        f = s.find(":")
        self.h = int(s[0:f])
        self.m = int(s[f+1:])

    def __add__(self, other):
        time = Time("00:00")
        time.h = self.h+other.h
        time.m = self.m+other.m
        time.h += int(time.m/60)
        time.m %= 60
        return time

    def __sub__(self, other):
        time = Time("00:00")
        time.h = self.h-other.h
        time.m = self.m-other.m
        time.h %= 24
        if time.m < 0:
            time.h -= 1
            time.m %= 60
        return time

    def show(self):
        print(f"{self.h}:{self.m}")

    def __str__(self):
        return str(self.h*60+self.m)


def main():
    cnt = Time("00:00")
    n = int(input())
    for i in range(n):
        a, b = input().split()
        cnt += (Time(b)-Time(a))
    print(cnt)
    # cnt.show()


if __name__ == "__main__":
    main()