#include <bits/stdc++.h>

using namespace std;

const int oo = 0x3f3f3f3f;
const double eps = 1e-9;
const double PI = 2.0 * acos(0.0);


typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<string> vs;

#define sz(c) int((c).size())
#define all(c) (c).begin(), (c).end()
#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define FORS(i,a,b,s) for (int i = (a); i < (b); i=i+(s))
#define FORD(i,a,b) for (int i = int(b)-1; i >= (a); i--)
#define FORIT(i,c) for (__typeof__((c).begin()) i = (c).begin(); i != (c).end(); i++)

#define MAXN 26

vi adj[MAXN]; // input: adjacency list
int p, v[MAXN]; // temp: node color
int fg; // output: cycle found (0/1)

int od[MAXN]; // output: nodes in order
void dfs(int a) {
	if(v[a]==1) fg = 1;
	if(v[a]) return;
	v[a] = 1; // gray
	FOR(i,0,sz(adj[a])) dfs(adj[a][i]);
	v[a] = 2; // black
	od[p] = a; p--;
}

void topsort(int n) {
	FOR(i,0,n) v[i]=0; //white
	fg=0; p=n-1;
	FOR(i,0,n) if(!v[i]) dfs(i);
}


int main(){
	int n; cin >> n;
	string cur = "";
	bool inval = false;
	FOR(i,0,n){
		string next; cin >> next;

		int diffPos = -1;
		FOR(j,0,min(sz(next), sz(cur))){
			if (next[j] != cur[j]) { diffPos = j; break; }
		}
		if (diffPos == -1){
			if (sz(next) < sz(cur)) inval = true;
		} else {
			adj[next[diffPos] - 'a'].push_back(cur[diffPos] - 'a');
		}

		cur = next;
	}

	topsort(26);

	if (fg || inval) cout << "impossible" << endl;
	else {
		FORD(i,0,26) cout << char('a' + od[i]);
		cout << endl;
	}
}