#include <bits/stdc++.h>
using namespace std;
vector<string> vec_splitter(string s) {
  for (char& c : s) c = c == ',' ? ' ' : c;
  stringstream ss;
  ss << s;
  vector<string> res;
  for (string z; ss >> z; res.push_back(z))
    ;
  return res;
}
void debug_out(vector<string> args, int idx) { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(vector<string> args, int idx, Head H, Tail... T) {
  if (idx > 0) cerr << ", ";
  stringstream ss;
  ss << H;
  cerr << args[idx] << " = " << ss.str();
  debug_out(args, idx + 1, T...);
}
void localTest() {}
const long long N = 1e6 + 1, M = 26;
string s[N], ans;
long long vis[M], inDeg[M];
vector<long long> V[M];
bool toposort() {
  queue<long long> Q;
  long long cnt = 0;
  for (long long i = 0; i < M; ++i) {
    if (vis[i]) {
      cnt++;
      if (!inDeg[i]) {
        Q.push(i);
      }
    }
  }
  long long alpha = 0;
  while (!Q.empty()) {
    auto pp = Q.front();
    Q.pop();
    ++alpha;
    ans += (char)('a' + pp);
    for (auto nxt : V[pp]) {
      inDeg[nxt]--;
      if (!inDeg[nxt]) {
        Q.push(nxt);
      }
    }
  }
  return alpha == cnt;
}
int main() {
  localTest();
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, k;
  string impos = "IMPOSSIBLE";
  cin >> n >> k;
  for (long long i = 0; i < n; ++i) {
    long long p;
    cin >> p;
    for (long long j = 0; j < k; ++j) {
      string word;
      cin >> word;
      s[k * p + j] = word;
      for (auto c : word) {
        vis[c - 'a'] = 1;
      }
    }
  }
  for (long long i = 0; i < n * k - 1; ++i) {
    bool fl = true;
    for (long long j = 0; j < (long long)min(s[i].size(), s[i + 1].size());
         ++j) {
      if (s[i][j] != s[i + 1][j]) {
        fl = false;
        long long fc = s[i][j] - 'a', sc = s[i + 1][j] - 'a';
        V[fc].push_back(sc);
        inDeg[sc]++;
        break;
      }
    }
    if (fl && s[i].size() > s[i + 1].size()) {
      cout << impos << "\n";
      return 0;
    }
  }
  if (toposort()) {
    cout << ans << "\n";
  } else {
    cout << impos << "\n";
  }
  return 0;
}
