from heapq import *
import sys

sys.setrecursionlimit(10 ** 6)
def MI(): return map(int, sys.stdin.readline().split())

def main():
    def stod(s):
        res = int(s[0]) * 10000 + int(s[2:4]) * 100 + int(s[5:7])
        return res

    n, m = MI()
    if n > m:
        print(m)
        exit()
    timeline = []
    for i in range(m):
        s = input()
        date0 = stod(s[:7])
        date1 = stod(s[8:])
        # [時間,0到着,退出時間,客番号]
        heappush(timeline, [date0, 0, date1, i])
    ans = 0
    stay = 0
    arrived = []
    leaved = [False] * m
    while timeline:
        date0, event, date1, i = heappop(timeline)
        # チェックアウト
        if event:
            if leaved[i]: continue
            # 追い出されずにチェックアウトできた人数が答え
            ans += 1
            stay -= 1
            leaved[i] = True
        # チェックイン
        else:
            heappush(arrived, [-date1, i])
            # [時間,1退出,なし,客番号]
            heappush(timeline, [date1, 1, -1, i])
            stay += 1
        # 収容人数を越えたら、チェックアウトが一番遅い客を追い出す
        if stay > n:
            while 1:
                date1, i = heappop(arrived)
                if not leaved[i]: break
            stay -= 1
            leaved[i] = True

    print(ans)

main()