3
rjnflsbpxqivrcdjptj
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
u
q
l
y
i
o
a
u
 5
splt
ohqykk
xqpz
knojbur
pmfm
 2
hk
obsp
 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
gtpr
kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
aycv
qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
o
qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
s
wpcsovfjlyspviflk
ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
zbi
q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
o
ubn
nzoc
h
wnjwjrf
t
bq
bw
 1
b

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

import java.io.*;
import java.math.*;
import java.text.*;
import java.util.*;

//Codeforces
public class MainCodeforces1 {
	private static MyScanner in;
	private static PrintStream out;

	public static void main(String[] args) throws IOException {
		// helpers for input/output
		boolean LOCAL_TEST = false;// change to false before submitting
		out = System.out;
		if (LOCAL_TEST) {
			in = new MyScanner("E:\\zin2.txt");
		}
		else {
			boolean usingFileForIO = false;
			if (usingFileForIO) {
				// using input.txt and output.txt as I/O
				in = new MyScanner("input.txt");
				out = new PrintStream("output.txt");
			}
			else {
				in = new MyScanner();
				out = System.out;
			}
		}

		solve();
	}

	private static void solve() throws IOException
	{
		int N = in.nextInt();
		String[] oldT = new String[N];
		for (int i = 0; i < N; i++) {
			oldT[i] = in.nextString();
		}

		String title = "a";
		while (true) {
			boolean isValid = true;
			for (int i = 0; i < N; i++) {
				if (oldT[i].contains(title)) {
					isValid = false;
					break;
				}
			}
			if (isValid)
				break;
			title = getNext(title);
		}
		out.println(title);
	}

	private static String getNext(String title) {
		char[] newT = title.toCharArray();
		int lastIdx = title.length() - 1;
		int x = lastIdx;
		while (x >= 0 && newT[x] == 'z') {
			x--;
		}
		if (x >= 0) {
			newT[x] += 1;
			if (x < lastIdx)
				newT[x + 1] = 'a';
			return new String(newT);
		}
		else {
			newT[lastIdx] = 'a';
			return (new String(newT)) + "a";
		}
	}

	// =====================================
	static class MyScanner {
		Scanner inp = null;

		public MyScanner() throws IOException
		{
			inp = new Scanner(System.in);
		}

		public MyScanner(String inputFile) throws IOException {
			inp = new Scanner(new FileInputStream(inputFile));
		}

		public int nextInt() throws IOException {
			return inp.nextInt();
		}

		public long nextLong() throws IOException {
			return inp.nextLong();
		}

		public double nextDouble() throws IOException {
			return inp.nextDouble();
		}

		public String nextString() throws IOException {
			return inp.next();
		}

	}

}