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import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;

public class NewProblem {
    public static void main(String[] args) throws Exception {
        PrintWriter pw = new PrintWriter(System.out);
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] in = br.readLine().split(" ");
        int n = Integer.parseInt(in[0]);
        String[] arr = new String[n];
        char[] ac1 = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
        char[] ac3 = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
        String[] ac2 = new String[676];
        int cnt = 0;
        for (int i = 0; i < n; i++) {
            arr[i] = br.readLine();
            for (int j = 0; j < arr[i].length(); j++) {
                for (int k = 0; k < 26; k++) {
                    if (arr[i].charAt(j) == ac1[k]) {
                        ac1[k] = '1';
                        cnt++;
                    }
                }
            }
        }
        if (cnt < 26) {
            for (int i = 0; i < 26; i++) {
                if (ac1[i] != '1') {
                    System.out.println(ac1[i]);
                    return;
                }
            }
        }
        else {
            for (int i = 0; i < 26; i++) {
                for (int j = 0; j < 26; j++) {
                    ac2[i * 26 + j] = ac3[i] + "" + ac3[j];
                }
            }
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < arr[i].length() - 1; j++) {
                    for (int k = 0; k < 676; k++) {
                        if ((arr[i].charAt(j) + "" + arr[i].charAt(j + 1)).equals(ac2[k])) {
                            ac2[k] = " ";
                        }
                    }
                }
            }
            for (int i = 0; i < 676; i++) {
                if (!ac2[i].equals(" ")) {
                    System.out.println(ac2[i]);
                    return;
                }
            }
        }
        pw.close();
    }
}
