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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;
public class code {
	public static void main(String[] args)throws IOException {
		FastReader sc = new FastReader();
		PrintWriter pw = new PrintWriter(System.out);
		int t = sc.nextInt();
		TreeSet<String> set = new TreeSet<>();
		TreeSet<Character> ad = new TreeSet<>();
		String d = "abcdefghijklmnopqrstuvwxyz";
		for (int i = 0; i < 26; i++) {
			ad.add(d.charAt(i));
		}
		for (int i = 0; i < 26; i++) {
			for (int j = 0; j < 26; j++) {
				String s = d.charAt(i) + "" + d.charAt(j);
				set.add(s);
			}
		}
		//System.out.println(set);
		String str[] = new String[t];
		String s = "";
		for (int i = 0; i < t; i++) {
			str[i] = sc.next();
			s = s + str[i];
		}

		// System.out.println(s);
		for (int i = 0; i < s.length(); i++) {
			if (ad.contains(s.charAt(i))) {
				ad.remove(s.charAt(i));
			}
		}
		boolean flag = false;
		if (ad.size() > 0) {
			flag = true;
			for (char i : ad) {
				System.out.println(i);
				break;
			}
		}
		if (!flag) {
			for (int i = 0; i < str.length; i++) {
				for (int j = 0; j < str[i].length() - 1; j++) {
					String g = str[i].charAt(j) + "" + str[i].charAt(j + 1);
					if (set.contains(g)) {
						set.remove(g);
					}
				}
			}

			for (String i : set) {
				System.out.println(i);
				break;
			}
		}

	}
}





class FastReader {
	BufferedReader br;
	StringTokenizer st;

	public FastReader() {
		br = new BufferedReader(new
		                        InputStreamReader(System.in));
	}

	String next() {
		while (st == null || !st.hasMoreElements()) {
			try {
				st = new StringTokenizer(br.readLine());
			} catch (IOException  e) {
				e.printStackTrace();
			}
		}
		return st.nextToken();
	}

	int nextInt() {
		return Integer.parseInt(next());
	}

	long nextLong() {
		return Long.parseLong(next());
	}

	double nextDouble() {
		return Double.parseDouble(next());
	}

	String nextLine() {
		String str = "";
		try {
			str = br.readLine();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return str;
	}
}