import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class NickyEnMark {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int n = scanner.nextInt();

		HashMap<Character, Set<Character>> pairings = new HashMap<>();
		Set<Character> dependant = new HashSet<>();

		String previous = null;
		for (int i = 0; i < n; i++) {
			String word = scanner.next();

			if (previous != null) {
				char[] previousLetters = previous.toCharArray();
				char[] secondLetters = word.toCharArray();
				int l = Math.max(previousLetters.length, secondLetters.length);

				for (int j = 0; j < l; j++) {
					if (j >= previousLetters.length) {
						continue;
					} else if (j >= secondLetters.length) {
						System.out.println("impossible");
						return;
					}

					char f = previousLetters[j];
					char s = secondLetters[j];

					if (f != s) {
						dependant.add(s);
						Set<Character> d;
						if (pairings.containsKey(f)) {
							d = pairings.get(f);
						} else {
							d = new HashSet<>();
							pairings.put(f, d);
						}

						d.add(s);
						break;
					}
				}
			}

			previous = word;
		}

		ArrayList<Character> order = new ArrayList<>();
		Set<Character> removed = new HashSet<>();

		for (char c = 'a'; c <= 'z'; c++) {
			if (!dependant.contains(c)) {
				order.add(c);
				removed.add(c);
			}
		}

		while (!pairings.isEmpty()) {
			Set<Character> removed2 = removed;
			removed = new HashSet<>();
			int numRemoved = 0;
			for (Character c : removed2) {
				if (pairings.containsKey(c)) {
					numRemoved++;
				}
				pairings.remove(c);
			}

			dependant = new HashSet<>();
			for (Character f : pairings.keySet()) {
				Set<Character> s = pairings.get(f);
				dependant.addAll(s);
			}

			for (char c = 'a'; c <= 'z'; c++) {
				if (!dependant.contains(c) && !order.contains(c)) {
					order.add(c);
					removed.add(c);
				}
			}

			if (numRemoved == 0) {
				System.out.println("impossible");
				return;
			}
		}

		for (Character c : order) {
			System.out.print(c);
		}

		System.out.println();
	}
}