N = int(input())

T = 0
for _ in range(N):
    X, Y = input().split()
    H1, M1 = map(int, X.split(':'))
    H2, M2 = map(int, Y.split(':'))
    T1 = H1*60 + M1
    T2 = H2*60 + M2
    T += (T2-T1+1440) % 1440

print(T)