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import java.io.InputStreamReader;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.TreeSet;
import java.util.StringTokenizer;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author ocelopilli
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		InputReader in = new InputReader(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskB solver = new TaskB();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskB {
    String giveMeString(int n)
    {
        String ret = "";

        while ( n > 0 )
        {
            n--;
            ret = (char)(n % 26 + 'a') + ret;
            n /= 26;
        }

        return ret;
    }
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n = in.nextInt();
        TreeSet <String> st = new TreeSet<String>();
        for (int i=0; i<n; i++)
        {
            String str = in.next();
            for (int j=0; j<str.length(); j++)
            {
                for (int k=1; j+k <= str.length(); k++)
                {
                    st.add( str.substring( j, j+k ) );
                }
            }
        }

        //for (int i=0; i<100; i++) out.println(i+" "+giveMeString(i));

        int i = 1;
        while ( true )
        {
            String x = giveMeString( i );
            if ( !st.contains( x ) )
            {
                out.println( x );
                break;
            }
            i++;
        }
    }
}

class InputReader {
    BufferedReader br;
    StringTokenizer st;

    public InputReader(InputStream in)
    {
        br = new BufferedReader(new InputStreamReader(in));
        st = null;
    }

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

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