#include <bits/stdc++.h>
using namespace std;
vector<vector<int>> g;
vector<bool> used, ok;
vector<int> ans;
int N = 26;
void dfs(int v) {
  used[v] = 1;
  for (int i = 0; i < g[v].size(); ++i) {
    int to = g[v][i];
    if (!used[to]) dfs(to);
  }
  ans.push_back(v);
}
void tops() {
  for (int i = 0; i < 26; ++i) {
    if (!ok[i]) continue;
    if (!used[i]) dfs(i);
  }
  reverse(ans.begin(), ans.end());
}
signed main() {
  std::ios_base::sync_with_stdio(0), cin.tie(0);
  int n, k;
  cin >> n >> k;
  vector<vector<string>> als(n);
  g.resize(26);
  used.resize(26);
  ok.resize(26);
  for (int i = 0; i < n; ++i) {
    int p;
    cin >> p;
    for (int j = 0; j < k; ++j) {
      string s;
      cin >> s;
      als[p].push_back(s);
      for (auto u : s) {
        ok[u - 'a'] = 1;
      }
    }
  }
  vector<string> st;
  for (auto u : als) {
    for (auto y : u) {
      st.push_back(y);
    }
  }
  for (int i = 1; i < st.size(); ++i) {
    int l = min(st[i].size(), st[i - 1].size());
    int j = 0;
    for (j = 0; j < l; ++j) {
      if (st[i][j] == st[i - 1][j])
        continue;
      else {
        int a1 = st[i - 1][j] - 'a', a2 = st[i][j] - 'a';
        g[a1].push_back(a2);
        break;
      }
    }
    if (j == l && st[i - 1].size() > st[i].size()) {
      cout << "IMPOSSIBLE";
      return 0;
    }
  }
  tops();
  used.assign(26, 0);
  for (auto i : ans) {
    for (auto u : g[i]) {
      if (used[u]) {
        cout << "IMPOSSIBLE";
        return 0;
      }
    }
    used[i] = 1;
  }
  string s;
  for (auto u : ans) {
    s += char('a' + u);
  }
  cout << s;
  return 0;
}
