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import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class Test {
	public static void main(String[] args) throws NumberFormatException,
			IOException {
		BufferedReader rd = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter pw = new PrintWriter(System.out);
		int n = Integer.parseInt(rd.readLine());
		// StringTokenizer st = new StringTokenizer(rd.readLine());
		String[] a = new String[n];
		for (int i = 0; i < n; i++)
			a[i] = rd.readLine();
		solve(a);
		pw.flush();
	}

	private static void solve(String[] a) {
		for (int i = 'a'; i <= 'z'; i++) {
			String tmp = "" + (char) i;
			if (!in(a, tmp)) {
				System.out.print(tmp);
				return;
			}
		}

		for (int i = 'a'; i <= 'z'; i++)
			for (int j = 'a'; j <= 'z'; j++) {
				String tmp = "" + (char) i + (char) j;
				if (!in(a, tmp)) {
					System.out.print(tmp);
					return;
				}
			}

		for (int i = 'a'; i <= 'z'; i++)
			for (int j = 'a'; j <= 'z'; j++) {
				for (int k = 'a'; k <= 'z'; k++) {
					String tmp = "" + (char) i + (char) j + (char) k;
					if (!in(a, tmp)) {
						System.out.print(tmp);
						return;
					}
				}
			}

		for (int i = 'a'; i <= 'z'; i++)
			for (int j = 'a'; j <= 'z'; j++) {
				for (int k = 'a'; k <= 'z'; k++) {
					for (int o = 'a'; o <= 'z'; o++) {
						String tmp = "" + (char) i + (char) j + (char) k
								+ (char) o;
						if (!in(a, tmp)) {
							System.out.print(tmp);
							return;
						}
					}
				}
			}

		for (int i = 'a'; i <= 'z'; i++)
			for (int j = 'a'; j <= 'z'; j++) {
				for (int k = 'a'; k <= 'z'; k++) {
					for (int o = 'a'; o <= 'z'; o++) {
						for (int l = 'a'; l <= 'z'; l++) {
							String tmp = "" + (char) i + (char) j + (char) k
									+ (char) o + (char)l;
							if (!in(a, tmp)) {
								System.out.print(tmp);
								return;
							}
						}
					}
				}
			}

	}

	private static boolean in(String[] a, String tmp) {
		for (int i = 0; i < a.length; i++)
			if (a[i].indexOf(tmp) != -1)
				return true;
		return false;
	}
}