#include<bits/stdc++.h>
using namespace std;
 
#define ll long long
 
 
// Convert string HH:MM to min
int convert(string s){
    int a=s[0]-48;
    int b=s[1]-48;
    int res=(a*10+b)*60;
    a=s[3]-48;
    b=s[4]-48;
    res+=(a*10)+b;
    return res;
}
 
void convert2(int n){
    int h=n/60;
    int min=n%60;
    if(h<=9) cout<<"0"<<h<<":";
    else cout<<h<<":";
    if(min<10)cout<<"0"<<min<<"\n";
    else cout<<min<<"\n";
}
 
void solve(){
    int n,k;
    cin>>n>>k;
    vector<int>time(1441,0);
    for(int i=0;i<n;i++){
        string a,b,s,e;
        cin>>a>>b>>s>>e;
        int t1=convert(s),t2=convert(e);
        time[t1]++;
        time[t2+1]--;
    }
    for(int i=1;i<1440;i++)time[i]+=time[i-1];
    int c=0;
    for(int i=0;i<1440;i++){
        if(time[i]==0){
            c++;
        }
        else c=0;
        if(c==k){
            convert2(i-k+1);
            return;
        }
    }
    cout<<-1<<"\n";
}
 
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    solve();
    return 0;
}
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