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import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Solver {

	public static void main(String[] Args) throws NumberFormatException,
			IOException {
		new Solver().Run();
	}

	PrintWriter pw;
	StringTokenizer Stok;
	BufferedReader br;

	public String nextToken() throws IOException {
		while (Stok == null || !Stok.hasMoreTokens()) {
			Stok = new StringTokenizer(br.readLine());
		}
		return Stok.nextToken();
	}

	public int nextInt() throws NumberFormatException, IOException {
		return Integer.parseInt(nextToken());
	}

	public double nextDouble() throws NumberFormatException, IOException {
		return Double.parseDouble(nextToken());
	}

	public long nextLong() throws NumberFormatException, IOException {
		return Long.parseLong(nextToken());
	}

	int n;
	String curStr;
	int[] usedChars;
	int[][] usedPairs;
	String result;
	final int offset=(int)'a';
	
	public void Run() throws NumberFormatException, IOException {
		//br = new BufferedReader(new FileReader("input.txt")); pw = new PrintWriter("output.txt");
		br=new BufferedReader(new InputStreamReader(System.in));  pw=new PrintWriter(new OutputStreamWriter(System.out));
		usedChars=new int[26];
		usedPairs=new int[26][26];
		int n=nextInt();
		for (int i=0; i<n; i++){
			curStr=br.readLine();
			int len=curStr.length();
			for (int j=0; j<len; j++){
				usedChars[curStr.charAt(j)-offset]++;
			}
			for (int j=0; j<len-1; j++){
				usedPairs[curStr.charAt(j)-offset][curStr.charAt(j+1)-offset]++;
			}
		}
		
		for (int i=0; i<26; i++){
			if (usedChars[i]==0){
				pw.println((char)(i+offset));
				pw.flush();
				pw.close();
				return;
			}
		}
		
		for (int i=0; i<26; i++){
			for (int j=0; j<26; j++){
				if (usedPairs[i][j]==0){
					pw.print((char)(i+offset));
					pw.println((char)(j+offset));
					pw.flush();
					pw.close();
					return;
				}
			}
		}
		
		pw.flush();
		pw.close();
	}

}
