3
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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

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

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author George Marcus
 */
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 {
    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int N = in.nextInt();
        String[] s = new String[N];
        for(int i = 0; i < N; i++)
            s[i] = in.nextString();

        // single letters
        int[] cnt = new int[26];
        for(int i = 0; i < N; i++)
            for(int j = 0; j < s[i].length(); j++)
                cnt[s[i].charAt(j) - 'a']++;

        int ch = -1;
        for(int i = 0; i < 26; i++)
            if(cnt[i] == 0) {
                ch = i;
                break;
            }

        if(ch != -1) {
            out.println((char)(ch + 'a'));
            return;
        }

        for(char a = 'a'; a <= 'z'; a++)
            for(char b = 'a'; b <= 'z'; b++) {
                boolean ok = true;
                for(int i = 0; i < N; i++)
                    for(int j = 0; j + 1 < s[i].length(); j++)
                        if(s[i].charAt(j) == a && s[i].charAt(j + 1) == b)
                            ok = false;
                if(ok) {
                    out.print(a);
                    out.print(b);
                    out.println();
                    return;
                }
            }
    }
}

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

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

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

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

    public String nextString() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuffer res = new StringBuffer();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

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

    }

