#include <bits/stdc++.h>
using namespace std;
using i64 = long long;

void solve() {
	int n, m;
	cin >> n >> m;
	vector<int> x(n);
	vector<string> t(n);
	for (int i = 0; i < n; i++) {
		cin >> x[i] >> t[i];
	}

	auto work = [&]() -> pair<vector<char>, int> {
		int n = x.size();
		vector<char> s(m + 1, '1');
		for (int i = 0; i < n; i++) {
			for (int k = x[i], p = 0; p < t[i].size(); k++, p++) {
				if (s[k] != '1' && s[k] != t[i][p]) {
					return {{}, -1};
				}
				s[k] = t[i][p];
			}
		}
		for (int i = 1; i <= m; i++) {
			if (s[i] == '1') {
				return {{}, -2};
			}
		}
		return {s, 0};
	};

	set<vector<char>> set;
	auto [s, y] = work();
	if (y == 0) {
		set.insert(s);
	}
	for (int i = 0; i < n; i++) {
		int xx = x[0];
		string tt = t[0];
		x.erase(x.begin());
		t.erase(t.begin());
		auto [s, y] = work();
		if (y == 0) {
			set.insert(s);
		}
		x.emplace_back(xx);
		t.emplace_back(tt);
	}

	if (set.size() > 1) {
		cout << "-2\n";
		return;
	}
	if (set.size() == 0) {
		cout << "-1\n";
		return;
	}

	auto print = [&](vector<char> s) {
		for (int i = 1; i <= m; i++) {
			cout << s[i];
		}
		cout << "\n";
	};
	print(*set.begin());
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);

	int t = 1;
	// cin >> t;

	while (t--) {
		solve();
	}

	return 0;
}