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 2
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 5
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 2
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 1
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 30
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 1
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import java.util.Scanner;

/**
 *
 * @author Izhari Ishak Aksa
 */
public class NewProblem {

    public static String inc(String s) {
        char[] temp = s.toCharArray();
        int sisa = 0;
        for (int i = s.length() - 1; i >= 0; i--) {
            int x = temp[i] - 'a';
            x += 1 + sisa;
            if (x > 25) {
                temp[i] = 'a';
                sisa = 1;
            } else {
                temp[i] = (char) (x + 'a');
                break;
            }
        }
        String ret = "";
        for (int i = 0; i < temp.length; i++) {
            ret += temp[i];
        }
        if (sisa == 1) {
            ret += "a";
        }
        return ret;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        String[] arr = new String[n];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.next();
        }
        String ret = "a";
        boolean ada = true;
        while (ada) {
            ada = false;
            for (int i = 0; i < n; i++) {
                if (arr[i].contains(ret)) {
                    ada = true;
                    break;
                }
            }
            if (ada) {
                ret = inc(ret);
            }
        }
        System.out.println(ret);
    }
}
