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import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.HashSet;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author pandusonu
 */
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();
    }

    static class TaskB {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            // out.print("Case #" + testNumber + ": ");
            int n = in.readInt();
            HashSet<String> xyz = new HashSet<>();
            for (int i = 0; i < n; i++) {
                String s = in.readString();
                for (int j = 0; j < s.length(); j++) {
                    for (int k = j + 1; k <= s.length(); k++) {
                        xyz.add(s.substring(j, k));
                    }
                }
            }
            String ans = "zzzz";
            for (int i = 1; i < 27; i++) {
                for (int j = 0; j < 27; j++) {
                    for (int k = 0; k < 27; k++) {
                        String s = "";
                        s += (char) ('a' + i - 1);
                        if (j != 0)
                            s += (char) ('a' + j - 1);
                        if (k != 0)
                            s += (char) ('a' + k - 1);
                        if (!xyz.contains(s)) {
                            if (s.length() < ans.length()) {
                                ans = s;
                            } else if (s.length() == ans.length()) {
                                if (ans.compareTo(s) > 0)
                                    ans = s;
                            }
                        }
                    }
                }
            }
            out.println(ans);
        }

    }

    static class InputReader {
        private final InputStream stream;
        private final byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        private int read() {
            try {
                if (curChar >= numChars) {
                    curChar = 0;
                    numChars = stream.read(buf);
                    if (numChars <= 0)
                        return -1;
                }
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
            return buf[curChar++];
        }

        public int readInt() {
            return (int) readLong();
        }

        public long readLong() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
                if (c == -1) throw new RuntimeException();
            }
            boolean negative = false;
            if (c == '-') {
                negative = true;
                c = read();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9') throw new InputMismatchException();
                res *= 10;
                res += (c - '0');
                c = read();
            } while (!isSpaceChar(c));
            return negative ? (-res) : (res);
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c)) c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.append((char) c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        private boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

    }
}

