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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
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import java.util.BitSet;
import java.util.Scanner;

public class CF278B {
	int n;
	String[] inputStr;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		CF278B cf = new CF278B();
		cf.n = sc.nextInt();
		cf.inputStr = new String[cf.n];
		for (int i = 0; i < cf.n; i++) {
			cf.inputStr[i] = sc.next();
		}
		System.out.println(cf.solve(cf.inputStr));
		sc.close();
	}

	public String solve(String[] inputStr) {
		BitSet bs = new BitSet();
		for (String str : inputStr) {
			char[] inputChar = str.toCharArray();
			for (char c : inputChar) {
				bs.set(c - 'a');
			}
		}
		int index = bs.nextClearBit(0);

		if (index < 26) {
			char c = (char) ('a' + index);
			return String.valueOf(c);
		} else {
			StringBuilder sb = new StringBuilder();
			for (int i = 0; i < 20; i++) {
				for (int m = 0; m < 26; m++) {
					char c = (char) ('a' + i);
					sb.append(c);
					for (int j = 0; j < 26; j++) {
						c = (char) ('a' + j);
						sb.append(c);
						boolean isSubstring = false;
						for (int k = 0; k < inputStr.length; k++) {
							if (inputStr[k].indexOf(sb.toString()) >= 0) {
								isSubstring = true;
								break;
							}
						}
						if (!isSubstring)
							return sb.toString();
						sb.deleteCharAt(sb.length() - 1);
					}
					sb.deleteCharAt(sb.length() - 1);
				}
			}
			return sb.toString();
		}

	}

}
