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
 z
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import java.util.Scanner;

public class _170_B {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);

		int n = in.nextInt();
		String[] str = new String[n];
		for (int i = 0; i < str.length; i++) {
			str[i] = in.next();
		}
		String ans = "";
		int len = 1;
		int cnt = 0;
		// bishtar az 3 ta nemishe!
		boolean b1 = false;
		boolean b2 = false;
		boolean exit = false;
		for (int i = -1; i < 26; i++) {
			if (b1)
				ans += (char) (i + 97);
			for (int j = -1; j < 26; j++) {
				if (b2)
					ans += (char) (j + 97);
				for (int j2 = 0; j2 < 26; j2++) {
					ans += (char) (j2 + 97);
					cnt = 0;
					for (int k = 0; k < str.length; k++) {
						if (str[k].contains(ans)) {
							break;
						} else
							cnt++;
					}
					if (cnt == n) {
						exit = true;
						break;
					} else
						ans = ans.substring(0, ans.length() - 1);
				}
				if (exit)
					break;
				else {
					b2 = true;
					if (ans.length() != 0)
						ans = ans.substring(0, ans.length() - 1);
				}
			}
			if (exit)
				break;
			else {
				b1 = true;
				if (ans.length() != 0)
					ans = ans.substring(0, ans.length() - 1);
			}
		}
		System.out.println(ans);

	}
}
