#include <iostream>
#include <algorithm>
#include <queue>
#include <vector>
#include <unordered_set>
#include <functional>
#include <stack>
#include <unordered_map>
#include <map>
#include <cstring>
using namespace std;

inline int read() {
    int x = 0, f = 1;
    char c = getchar();
    while (c > '9' || c < '0')
        f = c == '-' ? -1 : 1, c = getchar();
    while (c >= '0' && c <= '9')
        x = x * 10 + c - 48, c = getchar();
    return x * f;
}
using i64 = long long;
using u64 = unsigned long long;
using pii = pair<int, int>;
using pll = pair<i64, i64>;

const int maxn = 100 + 10;

string s;
int cnt[maxn];
int x[maxn];
string t[maxn];

bool insert(int pos, const string &t) {
    for (char c : t) {
        if (cnt[pos] and s[pos] != c)
            return false;
        cnt[pos]++;
        s[pos++] = c;
    }
    return true;
}

//void remove(int l, int r) {
//    for (int i = l; i <= r; i++) {
//        cnt[i]--;
//    }
//}

int main() {
    int n = read(), m = read();
    s.resize(m);
    vector<string> ans;
    bool flag = true;
    for (int i = 1; i <= n; i++) {
        x[i] = read() - 1;
        cin >> t[i];
        if (flag)
            flag &= insert(x[i], t[i]);
    }
    for (int j = 0; j < m; j++) {
        if (!cnt[j])
            flag = false;
    }
    if (flag) {
        ans.push_back(s);
    }
    for (int i = 1; i <= n; i++) {
        memset(cnt, 0, sizeof(cnt));
        bool flag = true;
        for (int j = 1; j <= n; j++) {
            if (i == j) continue;
            if (!insert(x[j], t[j])) {
                flag = false;
                break;
            }
        }
        for (int j = 0; j < m; j++) {
            if (!cnt[j])
                flag = false;
        }
        if (flag) {
            ans.push_back(s);
        }
    }
    if (ans.empty()) {
        cout << -1 << endl;
    } else if (ans.size() == 1) {
        cout << ans[0] << endl;
    } else {
        bool flag = true;
        for (int i = 1; i < ans.size(); i++) {
            if (ans[i] != ans[i - 1]) {
                flag = false;
                break;
            }
        }
        if (flag) {
            cout << ans[0] << endl;
        } else {
            cout << -2 << endl;
        }
    }
    return 0;
}