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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;
import java.util.StringTokenizer;



public class Round179_B {

    char[][]dict;
    int N;
    
    
    public void solve() throws Exception {

        InputReader in = new InputReader();
        N = in.nextInt();
        dict = new char[N][];
        for(int i =0 ; i < N;i++)
            dict[i] = in.next().toCharArray();
        boolean done1[] = new boolean [26];
        for(int i =0; i < N;i++){
            for(int j =0 ; j < dict[i].length;j++){
                int z = (dict[i][j]-'a');
                if(!done1[z])
                    done1[z] = true;
            }
        }
        for(int i =0 ; i < 26;i++)
            if(!done1[i])
            {
                System.out.println((char)(i+'a'));
                return;
            }
        boolean done2[][] = new boolean[26][26];
        for(int i =0 ;i < N;i++)
            for(int j =0;j < dict[i].length-1;j++){
                int x = (dict[i][j]-'a');
                int y = (dict[i][j+1]-'a');
                if(!done2[i][j])
                    done2[i][j] = true;
            }
        for(int i =0 ; i < 26;i++)
            for(int j =0 ; j < 26;j++)
                if(!done2[i][j]){
                    System.out.println((char)(i+'a') +""+(char)(j+'a'));
                    return;
                }
    }

    public static void main(String[] args) throws Exception {
        new Round179_B().solve();

    }

    static class InputReader {
        BufferedReader in;
        StringTokenizer st;

        public InputReader() throws IOException {
            in = new BufferedReader(new InputStreamReader(System.in));
            st = new StringTokenizer(in.readLine());

        }

        public String next() throws IOException {

            while (!st.hasMoreElements())
                st = new StringTokenizer(in.readLine());
            return st.nextToken();
        }

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

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

}
