#include <bits/stdc++.h>
using namespace std;
const int N = 3e6 + 5, M = 1e3 + 5;
const long long INF = 1e18 + 5;
inline long long read() {
  long long sum = 0, fh = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') fh = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    sum = sum * 10 + c - '0';
    c = getchar();
  }
  return sum * fh;
}
inline int read2() {
  char c = getchar();
  while (c != 'a' && c != 'b' && c != 'c' && c != '?') c = getchar();
  return c == '?' ? 0 : c - 'a' + 1;
}
inline int read3() {
  char c = getchar();
  while (c < 'a' || c > 'z') {
    c = getchar();
  }
  return c - 'a';
}
inline void write(long long x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
inline int gcd(int x, int y) { return y == 0 ? x : gcd(y, x % y); }
inline long long ab(long long x) { return x < 0 ? -x : x; }
inline long long fpow(long long qwe, long long asd, long long zxc) {
  if (asd < 0) return 0;
  long long a = qwe, b = 1, c = asd;
  while (c) {
    if (c & 1) b = b * a % zxc;
    a = a * a % zxc;
    c >>= 1;
  }
  return b;
}
int rd[31] = {0};
int sy[31] = {0};
vector<string> a[M];
int imap[31][31] = {0};
int ans[31], ac = 0;
char asd[M];
int main() {
  int n = read(), m = read();
  for (int i = 1; i <= n; ++i) {
    int x = read();
    for (int j = 1; j <= m; ++j) {
      string qwe;
      scanf("%s", &asd);
      qwe = (string)asd;
      for (int k = 0; k < qwe.length(); ++k) sy[qwe[k] - 'a' + 1] = 1;
      a[x].push_back(qwe);
    }
  }
  for (int i = 0; i < n; ++i) {
    if (i) {
      int x = a[i - 1].size() - 1;
      int imi = min(a[i - 1][x].length(), a[i][0].length());
      int kx = 1;
      for (int k = 0; k < imi; ++k) {
        if (a[i - 1][x][k] != a[i][0][k]) {
          if (!imap[a[i - 1][x][k] - 'a' + 1][a[i][0][k] - 'a' + 1])
            rd[a[i][0][k] - 'a' + 1] += 1;
          imap[a[i - 1][x][k] - 'a' + 1][a[i][0][k] - 'a' + 1] = 1;
          kx = 0;
          break;
        }
      }
      if (kx && a[i - 1][x].length() > a[i][0].length()) {
        puts("IMPOSSIBLE");
        return 0;
      }
    }
    for (int j = 1; j < a[i].size(); ++j) {
      int imi = min(a[i][j - 1].length(), a[i][j].length());
      int kx = 1;
      for (int k = 0; k < imi; ++k) {
        if (a[i][j - 1][k] != a[i][j][k]) {
          if (!imap[a[i][j - 1][k] - 'a' + 1][a[i][j][k] - 'a' + 1])
            rd[a[i][j][k] - 'a' + 1] += 1;
          imap[a[i][j - 1][k] - 'a' + 1][a[i][j][k] - 'a' + 1] = 1;
          kx = 0;
          break;
        }
      }
      if (kx && a[i][j - 1].length() > a[i][j].length()) {
        puts("IMPOSSIBLE");
        return 0;
      }
    }
  }
  queue<int> qwe;
  for (int i = 1; i <= 26; ++i) {
    if (!rd[i]) qwe.push(i);
  }
  while (!qwe.empty()) {
    int x = qwe.front();
    qwe.pop();
    ans[++ac] = x;
    for (int i = 1; i <= 26; ++i) {
      if (imap[x][i]) {
        --rd[i];
        if (!rd[i]) qwe.push(i);
      }
    }
  }
  if (ac != 26)
    puts("IMPOSSIBLE");
  else {
    for (int i = 1; i <= 26; ++i) {
      if (sy[ans[i]]) putchar(ans[i] + 'a' - 1);
    }
    cout << endl;
  }
}
