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

void executeTime() {
    cerr << "Time Taken: " << (float)clock() / CLOCKS_PER_SEC << " secs";
}

int main() {

    int n, k; cin >> n >> k;

    cin.ignore();

    vector<string> events(n);
    for (int i = 0; i < n; i++) {
        getline(cin, events[i], '\n');
    }

    vector<int> time(1440, 0);

    for (int i = 0; i < n; i++) {
        stringstream ss(events[i]);

        vector<string> v;

        string word;
        while (ss >> word) {
            v.push_back(word);
        }

        string h = v[2].substr(0, 2);
        string m = v[2].substr(3, 2);
        int st = stoi(h) * 60 + stoi(m);
        time[st]++;

        h = v[3].substr(0, 2);
        m = v[3].substr(3, 2);
        int en = stoi(h) * 60 + stoi(m);
        time[en + 1] -= 1;
    }



    for (int i = 1; i < 1440; i++) {
        time[i] += time[i - 1];
    }

    int i = 0, ans = -1, cnt = 0;

    for (int i = 0; i < 1440; i++) {
        if (time[i] == 0) {
            cnt++;

            if (cnt == k) {
                ans = i - cnt + 1;
                break;
            }


        } else {
            cnt = 0;
        }
    }

    cout << ans << endl;


    executeTime();
    return 0;
}