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import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.HashSet;
import java.util.Set;
import java.util.StringTokenizer;


public class BB {
	
	static StringTokenizer st;
	static BufferedReader in;
	public static void main(String[] args) throws IOException {
		in = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
		int n = nextInt();
		Set<String> set = new HashSet<String>();
		String[]s = new String[n+1];
		for (int i = 1; i <= n; i++) {
			s[i] = next();
		}
		for (int leng = 1; leng <= 3; leng++) {
			int[]a = new int[leng+1];
			set.clear();
			for (int i = 1; i <= n; i++) {
				for (int j = 0; j <= s[i].length()-leng; j++) {
					set.add(s[i].substring(j, j+leng));
				}
			}
			while (true) {
				String ss = "";
				for (int i = 1; i <= leng; i++) {
					ss += (char)(a[i]+97);
				}
				if (!set.contains(ss)) {
					System.out.println(ss);
					return;
				}
				int pos = leng;
				while (pos >= 1 && a[pos]==25) {
					a[pos] = 0;
					pos--;
				}
				if (pos==0)
					break;
				a[pos]++;
			}
		}
		pw.close();
	}
	private static int nextInt() throws IOException{
		return Integer.parseInt(next());
	}
	
	private static long nextLong() throws IOException{
		return Long.parseLong(next());
	}
	
	private static double nextDouble() throws IOException{
		return Double.parseDouble(next());
	}
	
	private static String next() throws IOException {
        while (st == null || !st.hasMoreTokens()) {
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }
}
