#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<ii> vii;

#define x first
#define y second
#define pb push_back
#define eb emplace_back
#define rep(i,a,b) for(auto i=(a); i<(b); ++i)
#define REP(i,n) rep(i,0,n)
#define all(v) (v).begin(), (v).end()
#define rs resize
#define DBG(x) cerr << __LINE__ << ": " << #x << " = " << (x) << endl

const ld PI = acos(-1.0);
template<class T> using min_queue =
priority_queue<T, vector<T>, greater<T>>;
template<class T> int sz(const T &x) {
    return (int) x.size(); // copy the ampersand(&)!
}

template<class T> ostream& operator<<(ostream &os, vector<T> &v) {
    os << "\n[";
    for(T &x : v) os << x << ',';
    return os << "]\n";
}

struct pairhash {
    public:
    template<typename T1, typename T2>
    size_t operator()(const pair<T1, T2> &p) const {
        size_t lhs = hash<T1>()(p.x);
        size_t rhs = hash<T2>()(p.y);
        return lhs ^ (rhs+0x9e3779b9+(lhs<<6)+(lhs>>2));
    }
};

vector<vector<char>> adj;
vector<char> alph;
vi visited;
bool impossible = false;

void dfs(char a) {
	if (visited[a-'a'] == 1) impossible = true;
	if (visited[a-'a'] != 0) return;
	visited[a-'a'] = 1;
	for (char b : adj[a-'a']) {
		dfs(b);
	}
	alph.pb(a);
	visited[a-'a'] = 2;
}


void run() {
	ll n;
	cin >> n;
	string prev = "";
	adj = vector<vector<char>>(26);
	visited = vi(26, 0);
	for (ll i = 0; i < n; i++) {
		string cur;
		cin >> cur;
		for (ll j = 0; j <= min(prev.size(), cur.size()); j++) {
			if (j == prev.size()) {
				continue;
			} else if (j == cur.size()) {
				cout << "impossible" << endl;
				return;
			}
			if (cur[j] != prev[j]) {
				adj[cur[j] - 'a'].pb(prev[j]);
				break;
			}
		}
		prev = cur;
	}
	for (char a = 'a'; a <= 'z'; a++) {
		dfs(a);
	}
	if (impossible) {
		cout << "impossible" << endl;
		return;
	}
	for (char a : alph) {
		cout << a;
	}
	cout << endl;
}

signed main() {
    // DON'T MIX "scanf" and "cin"!
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout << fixed << setprecision(20);
    run();
    return 0;
}