#include <bits/stdc++.h>   namespace P_TEMPLATE {   class fastin {    private:     int _ch, _f;       public:     inline fastin& operator>>(char& c) {       c = getchar();       return *this;     }     template <typename _Tp>     inline fastin& operator>>(_Tp& _x) {       _x = 0;       while (!isdigit(_ch)) _f |= (_ch == 45), _ch = getchar();       while (isdigit(_ch))         _x = (_x << 1) + (_x << 3) + (_ch ^ 48), _ch = getchar();       _f && (_x = -_x, _f = 0);       return *this;     }     fastin() { _ch = _f = 0; }   } fin;   class fastout {    private:     int _num[32], _head;       public:     inline fastout& operator<<(char c) {       putchar(c);       return *this;     }     template <typename _Tp>     inline fastout& operator<<(_Tp _x) {       _Tp _k;       if (_x == 0) {         putchar( 0 );         return *this;       }       if (_x < 0) putchar( - ), _x = -_x;       while (_x > 0)         _k = _x / 10, ++_head, _num[_head] = (_x - (_k << 1) - (_k << 3)) ^ 48,         _x = _k;       while (_head > 0) putchar(_num[_head]), --_head;       return *this;     }     fastout() { _head = 0; }   } fout;   inline void P_INIT() {}   }  // namespace P_TEMPLATE   using namespace P_TEMPLATE;   using namespace std;   const int maxn = 1000004, maxm = 104;   const int mod = 1000000007, inf = 1201201201;   int n, k, p;   char s[maxn][maxm];   int len[maxn];   int book[28], cnt[28];   vector<int> e[28];   queue<int> q;   int topo[28], indx;   inline bool check() {     for (register int i = 1; i <= 26; ++i)       if (!cnt[i]) q.push(i), topo[++indx] = i;     while (!q.empty()) {       int t = q.front();       q.pop();       for (register int i = 0; i < e[t].size(); ++i) {         --cnt[e[t][i]];         if (cnt[e[t][i]] == 0) q.push(e[t][i]), topo[++indx] = e[t][i];       }     }     return indx == 26;   }   signed main() {     P_INIT();     fin >> n >> k;     for (register int i = 1; i <= n; ++i) {       fin >> p;       for (register int j = 1; j <= k; ++j) {         scanf( %s , s[p * k + j] + 1), len[p * k + j] = strlen(s[p * k + j] + 1);         for (register int t = 1; t <= len[p * k + j]; ++t)           book[s[p * k + j][t] -  a  + 1] = 1;       }     }     for (register int i = 1; i < n * k; ++i) {       bool flag = 0;       for (register int j = 1; j <= std::min(len[i], len[i + 1]); ++j)         if (s[i][j] != s[i + 1][j]) {           e[s[i][j] -  a  + 1].push_back(s[i + 1][j] -  a  + 1);           ++cnt[s[i + 1][j] -  a  + 1];           flag = 1;           break;         }       if (!flag && len[i] > len[i + 1]) {         puts( IMPOSSIBLE );         return 0;       }     }     if (check()) {       for (register int i = 1; i <= 26; ++i)         if (book[topo[i]]) fout << char(topo[i] - 1 +  a );     } else       puts( IMPOSSIBLE );     return 0;   }   