import re
import heapq

N, M = map(int, input().split())
R = []
for _ in range(M):
    d_in, h_in, m_in, d_out, h_out, m_out = map(int, re.split('[ :]', input()))
    R.append([d_in * 1440 + h_in * 60 + m_in, d_out * 1440 + h_out * 60 + m_out])
R.sort(key=lambda x: x[1])
R.sort(key=lambda x: x[0])
pendings = []
count = 0
for r in R:
    heapq.heappush(pendings, (r[1], r[0]))
    while pendings[0][0] < r[0]:
        heapq.heappop(pendings)
        count += 1
    if len(pendings) > N:
        pendings.sort()
        pendings.pop()
        heapq.heapify(pendings)
count += len(pendings)
print(count)