#include <bits/stdc++.h>
using namespace std;
const long long N = 1005;
const long long M = 10005;
map<long long, string> mp;
string str[N * N];
long long n, k, m;
long long g[N][N], exist[N], vis[N], ind[N], fin[N], idx, seq[N], tot;
void dfs(long long p) {
  vis[p] = 1;
  for (long long q = 0; q < 26; q++) {
    if (vis[q] || !exist[q] || g[p][q] == 0) continue;
    dfs(q);
  }
  fin[p] = ++idx;
  seq[idx] = p;
}
signed main() {
  ios::sync_with_stdio(false);
  cin >> n >> k;
  for (long long i = 1; i <= n; i++) {
    long long p;
    cin >> p;
    for (long long j = 1; j <= k; j++) {
      string x;
      cin >> x;
      for (char c : x) exist[c - 'a'] = 1;
      mp[p * M + j] = x;
    }
  }
  for (auto i : mp) {
    ++m;
    str[m] = i.second;
  }
  for (long long i = 1; i < m; i++) {
    string s = str[i], t = str[i + 1];
    long long flag = 1;
    for (long long j = 0; j < min(s.length(), t.length()); j++) {
      if (s[j] != t[j]) {
        g[s[j] - 'a'][t[j] - 'a'] = 1;
        ind[t[j] - 'a']++;
        flag = 0;
        break;
      }
    }
    if (flag && s.length() > t.length()) {
      puts("IMPOSSIBLE");
      return 0;
    }
  }
  for (long long i = 0; i < 26; i++) {
    if (exist[i] && ind[i] == 0 && !vis[i]) dfs(i);
  }
  for (long long i = 0; i < 26; i++)
    if (exist[i]) ++tot;
  for (long long i = 0; i < 26; i++) {
    for (long long j = 0; j < 26; j++) {
      if (g[i][j]) {
        if (fin[i] <= fin[j]) {
          puts("IMPOSSIBLE");
          return 0;
        }
      }
    }
  }
  for (long long i = tot; i >= 1; i--) {
    cout << (char)(seq[i] + 'a');
  }
  cout << endl;
  return 0;
}
