/* -*- coding: utf-8 -*-
 *
 * 580.cc: No.580 旅館の予約計画 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100;
const int MAX_M = 1000;

/* typedef */

typedef pair<int,int> pii;
typedef multiset<pii> mspi;

/* global variables */

pii rs[MAX_M];
mspi rset;

/* subroutines */

inline int dhm2mnt(int d, int h, int m) { return (d * 24 + h) * 60 + m; }

bool ltmnt(const pii &a, const pii &b) {
  return a.second < b.second || (a.second == b.second && a.first < b.first);
}

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);

  for (int i = 0; i < m; i++) {
    int d0, h0, m0, d1, h1, m1;
    scanf("%d %d:%d %d %d:%d", &d0, &h0, &m0, &d1, &h1, &m1);
    rs[i].first = dhm2mnt(d1, h1, m1);
    rs[i].second = dhm2mnt(d0, h0, m0);
  }
  sort(rs, rs + m, ltmnt);

  int cnt = 0;

  for (int i = 0; i < m; i++) {
    pii &ri = rs[i];
    rset.insert(ri);

    while (! rset.empty() && rset.begin()->first < ri.second) {
      cnt++;
      rset.erase(rset.begin());
    }

    if (rset.size() > n) {
      mspi::iterator sit = rset.end();
      sit--;
      rset.erase(sit);
    }
  }

  cnt += rset.size();

  printf("%d\n", cnt);
  return 0;
}