#include<bits/stdc++.h>
#define int long long
#define inf 0x3f3f3f3f3f3f3f3f
#define maxn 1234567
#define eps 1e-7
#define mod 1000000007
#define Mod 998244353
#define f(i,a,b) for(int i=a;i<=b;i++)
#define r(i,a,b) for(int i=a;i>=b;i--) 
#define fx(i,a,b,x) for(int i=a;i<=b;i+=x)
#define rx(i,a,b,x) for(int i=a;i>=b;i-=x) 
using namespace std;
int T;
int n,M,d,h,m,cur[maxn],ans;
struct node{
	int l,r;
}a[maxn];
bool operator<(const node&a,const node&b){
	return a.r<b.r;
}
int Hash(int d,int h,int m){
	return d*1440+h*60+m;
}
int check(int l,int r){
	f(i,l,r)
		if(cur[i]==n)return 0;
	return 1;
}
priority_queue<string,vector<int>,greater<int>>pq;
void solve(){
	scanf("%lld%lld",&n,&M);
	f(i,1,M){
		scanf("%lld %lld:%lld",&d,&h,&m);
		a[i].l=Hash(d,h,m);
		scanf("%lld %lld:%lld",&d,&h,&m);
		a[i].r=Hash(d,h,m);
	}
	sort(a+1,a+M+1);
	f(i,1,M){
		while(!pq.empty()&&pq.top()<a[i].l)pq.pop();
		if(pq.size()<n&&check(a[i].l,a[i].r)==1){
			ans++;
			pq.push(a[i].r);
			f(j,a[i].l,a[i].r)cur[j]++;
		}
	}
	printf("%lld\n",ans);
}
signed main(){
	T=1;
//	scanf("%lld",&t);
	while(T--)solve();
	return 0;
}