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

const int N = 105;
int n, m;
bool ok[N];
pair<int, string> a[N];

int main() {
    cin >> n >> m;
    for (int i = 1; i <= n; i++) {
        cin >> a[i].first >> a[i].second;
    }
    sort(a + 1, a + n + 1);
    set<string> ans;
    for (int i = 1; i <= n; i++) {
        ok[i] = true;
        vector<pair<int, string>> v;
        for (int j = 1; j <= n; j++) {
            if (ok[j] == false) v.push_back({a[j].first, a[j].second});
        }
        ok[i] = false;
        bool check = true;
        string str = "";
        for (auto val : v) {
            int bg = val.first;
            string st = val.second;
            if (bg + st.size() > str.size()) {
                str.resize(bg + st.size(), ' ');
            }
            for (int j = 0; j < st.size(); j++) {
                if (str[bg + j] != ' ' && str[bg + j] != st[j])
                {
                    check = false;
                    break;
                }
                else
                str[bg + j] = st[j];
            }
        }
        if (check == true){
        string res = "";
        for (int j = 0; j < str.size(); j++)
            if (str[j] != ' ') res += str[j];
        if (res.size() == m) ans.insert(res);
        }
    }
    vector<pair<int, string>> vv;
        for (int j = 1; j <= n; j++) {
             vv.push_back({a[j].first, a[j].second});
        }
        bool check = true;
        string str = "";
        for (auto val : vv) {
            int bg = val.first;
            string st = val.second;
            if (bg + st.size() > str.size()) {
                str.resize(bg + st.size(), ' ');
            }
            for (int j = 0; j < st.size(); j++) {
                if (str[bg + j] != ' ' && str[bg + j] != st[j])
                {
                    check = false;
                    break;
                }
                else
                str[bg + j] = st[j];
            }
        }
        if (check == true){
        string res = "";
        for (int j = 0; j < str.size(); j++)
            if (str[j] != ' ') res += str[j];
        if (res.size() == m) ans.insert(res);
        }
    if (ans.size() == 1) cout << *ans.begin();
    else if (ans.size() == 0 ) cout << -1;
    else cout << -2;
}