#include <bits/stdc++.h>
using namespace std;

using ll =long long;

ll convert(string s){
	ll x=int(s[0])-48;
	ll y=int(s[1])-48;
	// cout<<x<<" "<<y<<endl;
	ll z=x*10+y;
	x=int(s[3])-48;
	y=int(s[4])-48;
	return z*60+x*10+y;
	
}

void ck(ll g){
	ll u=g/60;
	ll y=g%60;
	if(u<=9){
		cout<<"0";
		cout<<u;
	}
	else cout<<u;
	cout<<":";
	if(y<=9){
		cout<<"0";
		cout<<y<<endl;
	}
	else cout<<y<<endl;
}
int main() {
	// your code goes here
	ll  n;
	cin>>n;
	ll k;
	cin>>k;
	vector<ll>y(1441,0);
	for(ll i=0;i<n;i++){
		string a,b;
		cin>>a>>b;
		string u,v;
		cin>>u>>v;
		ll t1=convert(u);
		ll t2=convert(v);
		y[t1]=y[t1]+1;
		y[t2+1]=y[t2+1]-1;
		
	}
	ll i=1;
	while(i<=1440){
		y[i]=y[i]+y[i-1];
		i++;
	}
	int c=0,g=0;
	for(ll i=0;i<1440;i++){
		if(y[i]==0){
			c++;
			if(c==k){
				ck(i-k+1);
				g=1;
				break;
			}
		}
		else c=0;
	}
	
	if(g==0) cout<<-1<<endl;
	
	
	return 0;
}