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import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();

            boolean[] l1 = new boolean[26];
            boolean[][] l2 = new boolean[26][26];

            for (int it = 0; it < n; ++it) {
                String s = in.next();

                for (int i = 0; i < s.length(); ++i) {
                    char chr = s.charAt(i);
                    l1[chr - 'a'] = true;
                }

                for (int i = 1; i < s.length(); ++i) {
                    char chr1 = s.charAt(i - 1);
                    char chr2 = s.charAt(i);
                    l2[chr1 - 'a'][chr2 - 'a'] = true;
                }

            }


            for (int i = 0; i < 26; ++i) {
                if (!l1[i]) {
                    out.print((char) (i + 'a'));
                    return;
                }
            }

            for (int i = 0; i < 26; ++i) {
                for (int j = 0; j < 26; ++j) {
                    if (!l2[i][j]) {
                        out.print((char) (i + 'a') + "" + (char) (j + 'a'));
                        return;
                    }
                }
            }
        }

    }
}

