3
rjnflsbpxqivrcdjptj
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 1
z
 30
b
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f
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k
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a
c
w
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z
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
rxscdzkkezud
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 5
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abcdefghijklmnopqrst
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 1
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 10
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 5
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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xpbtbpftimnn
khbknqbvdkdd
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skrvnyz
avptgpjmenfzfxwckmt
 30
e
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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.IOException;
import java.util.Scanner;

public class TaskB {

	public static void main(String[] args) throws IOException {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		//sc.next();
		String[] a = new String[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.next();
		}
		String[] c = new String[100000];
		int l = 0;
		int r = 1;
		c[0] = "";
		for (char i = 'a'; i <= 'z'; i++) {
			c[r] = c[l] + i;
			r++;
		}
		l++;
		boolean b = true;

		while (b) {
			String s = c[l];
			b = false;
			for (int i = 0; i < n; i++) {
				if ((a[i].length() >= s.length()) && (a[i].indexOf(s) >= 0)) {
					b = true;
				}
			}
			for (char i = 'a'; i <= 'z'; i++) {
				c[r] = c[l] + i;
				r++;
				if (r >= 100000) {
					r = r % 100000;
				}
			}
			if (b) {
				l++;
				if (l >= 100000) {
					l = l % 100000;
				}
			}
		}

		System.out.println(c[l]);
	}

}