#include <bits/stdc++.h>
using namespace std;
const long long M = 1000000007;
bitset<26> used;
vector<vector<long long>> g(26);
vector<long long> topsort;
void dfs1(long long v) {
  used[v] = true;
  for (long long i = 0; i < (long long)g[v].size(); i++) {
    if (!used[g[v][i]]) {
      dfs1(g[v][i]);
    }
  }
  topsort.push_back(v);
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  long long n, k;
  cin >> n >> k;
  vector<string> W(n * k);
  vector<bool> bukva(26, false);
  for (long long i = 0; i < n; i++) {
    long long p;
    cin >> p;
    for (long long j = 0; j < k; j++) {
      cin >> W[p * n + j];
      for (auto c : W[p * n + j]) bukva[c - 'a'] = true;
    }
  }
  vector<pair<long long, long long>> l;
  for (long long i = 1; i < n * k; i++) {
    bool f = false;
    for (long long j = 0; j < min(W[i - 1].size(), W[i].size()); j++) {
      if (W[i - 1][j] != W[i][j]) {
        g[W[i][j] - 'a'].push_back(W[i - 1][j] - 'a');
        l.push_back({W[i][j] - 'a', W[i - 1][j] - 'a'});
        f = true;
        break;
      }
    }
    if (!f) {
      if (W[i - 1].size() > W[i].size()) {
        cout << "IMPOSSIBLE\n";
        exit(0);
      }
    }
  }
  for (long long i = 0; i < 26; i++) {
    if (bukva[i] && !used[i]) dfs1(i);
  }
  vector<long long> place(26);
  for (long long i = 0; i < topsort.size(); i++) {
    place[topsort[i]] = i;
  }
  for (auto p : l) {
    if (place[p.first] < place[p.second]) {
      cout << "IMPOSSIBLE\n";
      exit(0);
    }
  }
  for (auto v : topsort) cout << (char)('a' + v);
}
