Problem: While exploring the old caves, researchers found a book, or more precisely, a stash of mixed pages from a book. Luckily, all of the original pages are present and each page contains its number. Therefore, the researchers can reconstruct the book.

After taking a deeper look into the contents of these pages, linguists think that this may be some kind of dictionary. What's interesting is that this ancient civilization used an alphabet which is a subset of the English alphabet, however, the order of these letters in the alphabet is not like the one in the English language.

Given the contents of pages that researchers have found, your task is to reconstruct the alphabet of this ancient civilization using the provided pages from the dictionary.

Input

First-line contains two integers: n and k (1 ≤ n, k ≤ 10^3) — the number of pages that scientists have found and the number of words present at each page. Following n groups contain a line with a single integer p_i (0 ≤ n < 10^3) — the number of i-th page, as well as k lines, each line containing one of the strings (up to 100 characters) written on the page numbered p_i.

Output

Output a string representing the reconstructed alphabet of this ancient civilization. If the book found is not a dictionary, output "IMPOSSIBLE" without quotes. In case there are multiple solutions, output any of them.

Example

Input


3 3
2
b
b
bbac
0
a
aca
acba
1
ab
c
ccb


Output


acb

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using VI = vector<int>;
using VL = vector<ll>;
using VVI = vector<vector<int>>;
using VVL = vector<vector<ll>>;
using PII = pair<int, int>;
using PLL = pair<ll, ll>;
const double PI = acos(-1.0);
namespace Backlight {
const int __BUFFER_SIZE__ = 1 << 20;
bool NEOF = 1;
int __top;
char __buf[__BUFFER_SIZE__], *__p1 = __buf, *__p2 = __buf, __stk[996];
template <typename T>
T MIN(T a, T b) {
  return min(a, b);
}
template <typename First, typename... Rest>
First MIN(First f, Rest... r) {
  return min(f, MIN(r...));
}
template <typename T>
T MAX(T a, T b) {
  return max(a, b);
}
template <typename First, typename... Rest>
First MAX(First f, Rest... r) {
  return max(f, MAX(r...));
}
template <typename T>
void updMin(T &a, T b) {
  if (a > b) a = b;
}
template <typename T>
void updMax(T &a, T b) {
  if (a < b) a = b;
}
inline char nc() {
  return __p1 == __p2 && NEOF &&
                 (__p2 =
                      (__p1 = __buf) + fread(__buf, 1, __BUFFER_SIZE__, stdin),
                  __p1 == __p2)
             ? (NEOF = 0, EOF)
             : *__p1++;
}
template <typename T>
inline bool read(T &x) {
  char c = nc();
  bool f = 0;
  x = 0;
  while (!isdigit(c)) c == '-' && (f = 1), c = nc();
  while (isdigit(c)) x = (x << 3) + (x << 1) + (c ^ 48), c = nc();
  if (f) x = -x;
  return NEOF;
}
inline bool need(char c) { return (c != '\n') && (c != ' '); }
inline bool read(char &a) {
  while ((a = nc()) && need(a) && NEOF)
    ;
  return NEOF;
}
inline bool read(char *a) {
  while ((*a = nc()) && need(*a) && NEOF) ++a;
  *a = '\0';
  return NEOF;
}
inline bool read(double &x) {
  bool f = 0;
  char c = nc();
  x = 0;
  while (!isdigit(c)) {
    f |= (c == '-');
    c = nc();
  }
  while (isdigit(c)) {
    x = x * 10.0 + (c ^ 48);
    c = nc();
  }
  if (c == '.') {
    double temp = 1;
    c = nc();
    while (isdigit(c)) {
      temp = temp / 10.0;
      x = x + temp * (c ^ 48);
      c = nc();
    }
  }
  if (f) x = -x;
  return NEOF;
}
template <typename First, typename... Rest>
inline bool read(First &f, Rest &...r) {
  read(f);
  return read(r...);
}
template <typename T>
inline void print(T x) {
  if (x < 0) putchar('-'), x = -x;
  if (x == 0) {
    putchar('0');
    return;
  }
  __top = 0;
  while (x) {
    __stk[++__top] = x % 10 + '0';
    x /= 10;
  }
  while (__top) {
    putchar(__stk[__top]);
    --__top;
  }
}
template <typename First, typename... Rest>
inline void print(First f, Rest... r) {
  print(f);
  putchar(' ');
  print(r...);
}
template <typename T>
inline void println(T x) {
  print(x);
  putchar('\n');
}
template <typename First, typename... Rest>
inline void println(First f, Rest... r) {
  print(f);
  putchar(' ');
  println(r...);
}
template <typename T>
inline void _dbg(const char *format, T value) {
  cerr << format << '=' << value << endl;
}
template <typename First, typename... Rest>
inline void _dbg(const char *format, First f, Rest... r) {
  while (*format != ',') cerr << *format++;
  cerr << '=' << f << ", ";
  _dbg(format + 1, r...);
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> V) {
  os << "[ ";
  for (auto v : V) os << v << ",";
  return os << " ]";
}
template <typename T>
ostream &operator<<(ostream &os, set<T> V) {
  os << "[ ";
  for (auto v : V) os << v << ",";
  return os << " ]";
}
template <typename T>
ostream &operator<<(ostream &os, multiset<T> V) {
  os << "[ ";
  for (auto v : V) os << v << ",";
  return os << " ]";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, map<T1, T2> V) {
  os << "[ ";
  for (auto v : V) os << v << ",";
  return os << " ]";
}
template <typename L, typename R>
ostream &operator<<(ostream &os, pair<L, R> P) {
  return os << "(" << P.first << "," << P.second << ")";
}
}  // namespace Backlight
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l, int r) { return l + rng() % (r - l + 1); }
const int N = 5e5 + 5;
const int M = 3e6 + 5;
const int K = 1e7 + 5;
const int MOD = 1e9 + 7;
const int INF = 0x3f3f3f3f;
const ll LLINF = 0x3f3f3f3f3f3f3f3f;
const double eps = 1e-8;
void solve(int Case) {
  int n, k;
  cin >> n >> k;
  VI used(26);
  vector<string> ss(n * k);
  for (int i = (0); i < (n); ++i) {
    int p;
    cin >> p;
    for (int j = (0); j < (k); ++j) {
      cin >> ss[p * n + j];
      for (const auto &c : ss[p * n + j]) used[c - 'a'] = 1;
    }
  }
  int alpha = 0;
  for (int v : used)
    if (v) ++alpha;
  VVI G(26);
  VI deg(26);
  auto fail = []() {
    puts("IMPOSSIBLE");
    exit(0);
  };
  auto add = [&](int i, int j) {
    int n = (int((ss[i]).size())), m = (int((ss[j]).size()));
    for (int _ = (0); _ < (min(n, m)); ++_) {
      if (ss[i][_] != ss[j][_]) {
        int u = ss[i][_] - 'a', v = ss[j][_] - 'a';
        G[u].push_back(v);
        ++deg[v];
        return;
      }
    }
    if (n > m) fail();
  };
  int j = -1;
  for (int i = (0); i < (n * k); ++i)
    if ((int((ss[i]).size()))) {
      if (j != -1) add(j, i);
      j = i;
    }
  string t;
  queue<int> q;
  for (int i = (0); i < (26); ++i)
    if (used[i] == 1 && deg[i] == 0) q.push(i);
  while (!q.empty()) {
    int u = q.front();
    q.pop();
    t += char(u + 'a');
    for (int v : G[u]) {
      --deg[v];
      if (deg[v] == 0) q.push(v);
    }
  }
  if ((int((t).size())) != alpha) fail();
  cout << t << endl;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int T = 1;
  for (int _ = 1; _ <= T; _++) solve(_);
  return 0;
}