3
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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
b

a
 a
 e
 a
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 aa
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 ab
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 a


import java.util.Scanner;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
/**
 *
 * @author AltynbeKZ
 */
public class b {

    public static int k = 0;

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        char[] f = new char[50];
        f[0] = 96;

        int n = in.nextInt();
        String[] s = new String[n];

        for (int i = 0; i < n; i++) {
            s[i] = in.next();
        }

        int x = 97;
        boolean iscontain = true;
        String ss = "";

        while (iscontain) {
            ss = toString(f);
            iscontain = false;
            for (int i = 0; i < n; i++) {
                if (s[i].contains(ss)) {
                    iscontain = true;
                    break;

                }
            }
        }
        System.out.println(ss);
    }

    public static String toString(char[] f) {
        String s = "";

        f[0] += 1;
        for (int i = 0; i <= k; i++) {
            if (f[i] == 123) {
                f[i + 1] += 1;
                f[i] = 97;
            }
            s = ((char) f[i]) + s;
        }
        if (f[k + 1] != 0) {
            f[++k] = 97;
            s = ((char) f[k]) + s;
        }
        return s;
    }
}
