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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;

public class B {

	public static class node implements Comparable<node> {
		String s;

		@Override
		public int compareTo(node o) {
			if (s.length() < o.s.length())
				return -1;
			if (s.length() > o.s.length())
				return 1;
			return s.compareTo(o.s);
		}
	}

	static HashSet<String> v = new HashSet<String>();
	static PrintWriter out = new PrintWriter(System.out);
	static ArrayList<node> ans = new ArrayList<node>();

	static void solve(String s) {
		if (!v.contains(s)) {
			node tt = new node();
			tt.s = s;
			ans.add(tt);
			return;
		}
		for (int i = 0; i < 26; i++) {
			String t = s;
			t += (char) (i + 'a');
			solve(t);
		}
	}

	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String[] s = new String[n];
		for (int i = 0; i < s.length; i++)
			s[i] = br.readLine();
		for (int i = 0; i < s.length; i++)
			for (int j = 0; j < s[i].length(); j++)
				for (int k = j; k <= s[i].length(); k++)
					v.add(s[i].substring(j, k));
		int[] a = new int[26];
		for (int i = 0; i < s.length; i++)
			for (int j = 0; j < s[i].length(); j++)
				a[s[i].charAt(j) - 'a']++;
		boolean done = false;
		for (int i = 0; i < a.length; i++) {
			if (a[i] == 0) {
				out.println((char) (i + 'a'));
				done = true;
				break;
			}
		}
		if (!done) {
			solve("");
			Collections.sort(ans);
			out.println(ans.get(0).s);
		}
		out.close();
	}

}
