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// @author Sanzhar
import java.io.*;
import java.util.*;
import java.awt.Point;

public class Template {

    BufferedReader in;
    PrintWriter out;
    StringTokenizer st;

    String next() {
        while (st == null || !st.hasMoreTokens()) {
            try {
                st = new StringTokenizer(in.readLine());
            } catch (Exception e) {
            }
        }
        return st.nextToken();
    }

    int nextInt() {
        return Integer.parseInt(next());
    }

    long nextLong() {
        return Long.parseLong(next());
    }

    double nextDouble() {
        return Double.parseDouble(next());
    }

    public void run() throws Exception {
        //in = new BufferedReader(new FileReader("input.txt"));
        //out = new PrintWriter(new FileWriter("output.txt"));
        in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);
        solve();
        out.flush();
        out.close();
        in.close();
    }

    public void solve() throws Exception {
        int n = nextInt();
        boolean[][] t = new boolean[27][27];
        for (int i = 0; i < n; i++) {
            char[] c = next().toCharArray();
            for (int j = 0; j < c.length; j++) {
                int y = c[j] - 'a' + 1;
                t[0][y] = true;
                if (j > 0) {
                    int x = c[j - 1] - 'a' + 1;
                    t[x][y] = true;
                }
            }
        }
        for (int i = 0; i <= 26; i++) {
            for (int j = 1; j <= 26; j++) {
                if (!t[i][j]) {
                    if (i > 0) {
                        out.print(((char) (i + 'a' - 1)) + "");
                    }
                    out.println("" + ((char) (j + 'a' - 1)));
                    return;
                }
            }
        }
    }

    public static void main(String[] args) throws Exception {
        new Template().run();
    }
}
