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

public class Task228B {

	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(in.readLine());
		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = in.readLine();
		}
		
		HashSet<String> h = new HashSet<>();
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < s[i].length(); j++) {
				for (int k = j + 1; k <= s[i].length(); k++) {
					h.add(s[i].substring(j, k));
				}
			}
		}
		
		int c = 1;
		while (true) {			
			int ct = c;
			StringBuilder e = new StringBuilder();
			do {
				e.append((char)(ct % 27 + 96)); 
				ct /= 27;
			} while (ct != 0);
			String es = e.reverse().toString();
			if (!h.contains(es)) {
				System.out.println(es);
				return;
			}
			c++;
			if (c % 27 == 0) { c++; }
		}
	}
}