#include <bits/stdc++.h>
using namespace std;
template <class T>
ostream &operator<<(ostream &os, const vector<T> &p) {
  os << "[";
  for (auto &it : p) os << it << " ";
  return os << "]";
}
template <class T>
ostream &operator<<(ostream &os, const set<T> &p) {
  os << "[";
  for (auto &it : p) os << it << " ";
  return os << "]";
}
template <class T>
ostream &operator<<(ostream &os, const multiset<T> &p) {
  os << "[";
  for (auto &it : p) os << it << " ";
  return os << "]";
}
template <class S, class T>
ostream &operator<<(ostream &os, const map<S, T> &p) {
  os << "[";
  for (auto &it : p) os << it << " ";
  return os << "]";
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << "(" << p.first << "," << p.second << ")";
}
template <class S, class T>
ostream &operator<<(ostream &os, const unordered_map<S, T> &v) {
  for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' ';
  return os;
}
template <class T>
bool umin(T &a, T b) {
  return a > b ? (a = b, true) : false;
}
template <class T>
bool umax(T &a, T b) {
  return a < b ? (a = b, true) : false;
}
const long long N = 2e5 + 10;
const long long mod = 1e9 + 7;
const long long max6 = 1e6 + 6;
const long long oo = 1e9 + 9;
vector<long long> topo;
long long cl[max6], page[max6], mark[max6];
bool ok = false;
string s[1003][1005];
string ss[max6];
vector<long long> g[max6];
long long n, k;
void dfs(long long u) {
  cl[u] = 1;
  for (auto &v : g[u])
    if (cl[v] == 1)
      ok = false;
    else if (cl[v] == 0)
      dfs(v);
  cl[u] = 2;
  topo.push_back(u);
}
void solve(long long tc) {
  cin >> n >> k;
  for (long long i = 1; i <= n; ++i) {
    cin >> page[i];
    for (long long j = 1; j <= k; ++j) cin >> s[page[i]][j];
  }
  long long sl = 0;
  for (long long i = 0; i < n; ++i)
    for (long long j = 1; j <= k; ++j) ss[++sl] = s[i][j];
  for (long long i = 1; i <= sl; ++i)
    for (long long j = 0; j < ss[i].size(); ++j) mark[ss[i][j] - 'a'] = 1;
  ok = true;
  for (long long i = 2; i <= sl; ++i) {
    long long len = min(ss[i].size(), ss[i - 1].size());
    for (long long j = 0; j < len; ++j)
      if (ss[i][j] != ss[i - 1][j]) {
        long long u = ss[i - 1][j] - 'a';
        long long v = ss[i][j] - 'a';
        g[u].push_back(v);
        goto l1;
      }
    if (ss[i].size() < ss[i - 1].size()) ok = false;
  l1:;
  }
  for (long long i = 0; i < 26; ++i)
    if (mark[i] && cl[i] == 0) dfs(i);
  if (!ok) {
    cout << "IMPOSSIBLE";
    return;
  }
  reverse(topo.begin(), topo.end());
  for (auto &v : topo) {
    char res = 'a' + v;
    cout << res << "";
  }
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  for (long long i = 1; i <= t; i++) solve(i);
  return 0;
}
