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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

public class BB {

	// static int k = 0;
	// static int[] array;
	static String[] s;
	// ArrayList<String> last = new ArrayList<String>(90000);
	static String out = null;

	static boolean contain(String ss, String d) {
		int l1 = d.length(), l = ss.length();
		if (l1 > l)
			return false;
		for (int i = 0; i < l; i++) {
			boolean e = true;
			for (int j = 0; j < l1; j++) {
				try {
					if (d.charAt(j) != ss.charAt(i)) {
						e = false;
						break;
					}
				} catch (Exception e2) {
					// TODO: handle exception
					e = false;
					break;
				}

			}
			if (e) {
				return true;
			}
		}
		return false;
	}

	public static void perm(byte[] use, int i) {
		if (i > 0) {
			String cur = new String(use);
			cur = cur.substring(0, i);
			boolean found = false;
			int l = s.length;
			for (int j = 0; j < l; j++) {
				if (s[j].contains(cur)) {
					found = true;
					break;
				}
			}
			if (!found) {
				// System.out.println(cur);

				if (out == null || cur.length() < out.length()
						|| cur.compareTo(out) < 0)
					out = cur;
				// System.exit(0);
				// if (cur.compareTo("j") == 0)
				// System.out.println(cur + "     " + out);
			}
			if (i == 3)
				return;
			// int sum = totMov;
			// for (int j = 0; j < 3; j++)
			// if (use[j] == 'B')
			// sum -= map[j][0];
			// else if (use[j] == 'C')
			// sum -= map[j][2];
			// else
			// sum -= map[j][1];
			// if (sum < moves) {
			// moves = sum;
			// out = use.clone();
			// }
			// return;

		}
		for (int j = 0; j < 26; j++) {
			{
				// used[j] = true;
				use[i] = (byte) (j + 'a');
				perm(use, i + 1);
				// used[j] = false;
				use[i] = '\0';
			}
		}
	}

	public static void main(String[] args) throws NumberFormatException,
			IOException {
		// System.out.println();
		BufferedReader rd = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(rd.readLine());
		s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = rd.readLine();
		}
		byte use[] = new byte[] { '\0', '\0', '\0' };
		perm(use, 0);
		System.out.println(out);
	}

}
