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 1
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import java.util.Scanner;

public class B {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        sc.nextLine();
        String[] a = new String[n];
        for (int i = 0; i < n; i++)
            a[i] = sc.nextLine();
        boolean flag = true;
        int[] sub = new int[1];
        sub[0] = 97;
        int index = 97;
        int pos = 1;
        while (flag) {
            flag = false;
            for (int i = 0; i < n; i++) {
                if (contains(a[i], sub)) {
                    flag = true;
                    break;
                }
            }
            if (flag) {
                int t;
                for (t = pos - 1; t >= 0; t--)
                    if (sub[t] < 122) {
                        sub[t]++;
                        break;
                    }
                if (t < 0) {
                    sub = new int[pos + 1];
                    pos++;
                    for (int i = 0; i < pos; i++)
                        sub[i] = 97;
                } else
                    for (int i = t + 1; i < pos; i++)
                        sub[i] = 97;

            }
        }
        for (int i = 0; i < sub.length; i++)
            System.out.print((char) sub[i]);

    }

    public static boolean contains(String str, int[] c) {
        char[] b = str.toCharArray();
        int n = b.length;
        int m = c.length;
        boolean flag = true;
        for (int i = 0; i < n; i++) {
            if (i + m > n)
                return false;
            flag = true;
            for (int j = i, k = 0; k < m; j++, k++)
                if (b[j] != (char) c[k]) {
                    flag = false;
                    break;
                } else
                    flag = true;
            if (flag)
                return true;
        }
        return flag;
    }

}