import time
from datetime import datetime, timedelta

def main():
    N = int(input())
    T = tuple(tuple(input().split()) for _ in [0] * N)

    for t1, t2 in T:
        bed = datetime.strptime(t1, '%H:%M')
        rising = datetime.strptime(t2, '%H:%M')
        x = rising - bed
        if 'sleep' in locals():
            sleep = sleep + x
        else:
            sleep = x

    x = str(sleep).split()[2]
    h, m, s = map(int, x.split(':'))

    print(h * 60 + m)

main()