3
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vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
u
p
n
f
m
t
i
j
k
p
h
a
c
w
z
z
p
t
d
o
w
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q
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y
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o
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 5
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 2
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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kjexeesevrlowxhghq
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jrsrvsvbaiumlmkptn
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szahigtmklglrcocbo
znligfxkgxzkcfeu
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xgrxkgiehxztv
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jrdgajschhwsci
doxnxbjwptnimjmsuijx
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
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jos
qy
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nayke
ge
s
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t
bq
bw
 1
b

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

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Task278B {

	public static void main(String... args) throws NumberFormatException,
			IOException {
		Solution.main(System.in, System.out);
	}

	static class Solution {
		public static void main(InputStream is, OutputStream os)
				throws NumberFormatException, IOException {
			PrintWriter pw = new PrintWriter(os);
			Scanner s = new Scanner(is);

			int n = s.nextInt();

			Set<String> titles = new TreeSet<>();
			for (int i = 0; i < n; i++) {
				titles.add(s.nextLine());
			}

			String retVal = "";

			for (char lit = 'a'; lit <= 'z'; lit++) {
				boolean ok = true;
				for (String title : titles) {
					if (title.indexOf("" + lit) >= 0) {
						ok = false;
						break;
					}
				}
				if (ok) {
					retVal += lit;
					break;
				}
			}

			pw.println(retVal);

			pw.flush();
			s.close();
		}
	}

}
