/* Topcoder SRM 705, D2 500-Pointer.
   AlphabetOrderDiv2
   James S. Plank
   Wed Oct  6 14:05:31 EDT 2021
 */

#include <string>
#include <vector>
#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

class AlphabetOrderDiv2 {
  public:
    string isOrdered(vector <string> words);
};

string AlphabetOrderDiv2::isOrdered(vector <string> words)
{
  int i, j, c1, c2;
  vector < vector <int> > Adj;
  vector <int> nincident;
  vector <int> inc_nodes;

  /* Create an adjacency matrix from all of the words. 
     You should make sure that you don't set an edge from a node to itself.  */

  Adj.resize(26);
  for (i = 0; i < Adj.size(); i++) Adj[i].resize(26, 0);

  for (i = 0; i < words.size(); i++) {
    for (j = 1; j < words[i].size(); j++) {
      c1 = words[i][j-1] - 'a';
      c2 = words[i][j] - 'a';
      if (c1 != c2) Adj[c1][c2] = 1;
    }
  }

  /* Print the adjacency matrix. */

//   printf("  ");
//   for (i = 'a'; i <= 'z'; i++) printf("%c", i);
//   printf("\n");
//   for (i = 0; i < Adj.size(); i++) {
//     printf("%c ", 'a'+i);
//     for (j = 0; j < Adj.size(); j++) printf("%d", Adj[i][j]);
//     printf("\n");
//   }
 
  /* For each edge in the adjacency matrix, add one to the number of incident edges
     for each node. */

  nincident.resize(Adj.size(), 0);
  for (i = 0; i < Adj.size(); i++) {
    for (j = 0; j < Adj[i].size(); j++) {
      if (Adj[i][j]) nincident[j]++;
    }
  }

  /* Now make a vector of all of the nodes with no incident edges: */

  for (i = 0; i < nincident.size(); i++) if (nincident[i] == 0) inc_nodes.push_back(i);
  
  /* Do the topological sort -- run through inc_nodes and decrement nincident for each node
     incident to the node in question.  If that becomes zero, push it back onto inc_nodes. */

  for (i = 0; i < inc_nodes.size(); i++) {
    c1 = inc_nodes[i];
    for (j = 0; j < Adj[c1].size(); j++) {
      if (Adj[c1][j] == 1) {
        nincident[j]--;
        if (nincident[j] == 0) inc_nodes.push_back(j);
      }
    }
  }

  /* If all of the nodes now have 0 incident edges, the graph was acyclic.  There are 26
     nodes, so simply check whether inc_nodes.size() is 26. */

  return (inc_nodes.size() == 26) ? "Possible" : "Impossible";
}