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.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintStream;
import java.io.BufferedOutputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;

public class Main {

    public static void main(String[] args) {
        if (args.length > 0 && args[0].toLowerCase().equals("local")) {
            try {
                System.setIn(new FileInputStream("input.txt"));
                System.setOut(new PrintStream(new File("output.txt")));
            } catch (IOException exc) {
            }
        }
        Task task = new Task();
        task.solve();
    }

    public static class Task {
        static final long MOD = (long) 1e9 + 7;
        static final PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));

        public void solve() {
            try {
                Scan sc = new Scan();
                int n = (int) sc.scanLong();
                // pegionhole principle

                int[] twos = new int[730];
                int[] ones = new int[27];

                for (int i = 0; i < n; ++i) {
                    char[] curr = sc.scanString().toCharArray();
                    for (int j = 0, end = curr.length; j < end; ++j) {
                        int c = curr[j] - 'a';
                        ones[c + 1] = 1;
                        if (j > 0) {
                            int hash = (curr[j - 1] - 'a' + 1) * 27 + (c + 1);
                            twos[hash] = 1;
                        }
                    }
                }

                for (int i = 1; i < 27; ++i) {
                    if (ones[i] == 0) {
                        out.println((char) ('a' + i - 1));
                        return;
                    }
                }

                for (int i = 1; i < 27; ++i) {
                    for (int j = 1; j < 27; ++j) {
                        int hash = i * 27 + j;
                        if (twos[hash] == 0) {
                            out.println((char) ('a' + i - 1) + "" + (char) ('a' + j - 1));
                            return;
                        }
                    }
                }
            } finally {
                out.close();
            }
        }

        private void debug(Object... x) {
            StringBuilder sb = new StringBuilder();
            for (Object o : x)
                sb.append(String.valueOf(o)).append(" ");
            out.println(sb);
        }
    }

    public static class Scan {
        private byte[] buf = new byte[1024];
        private int index;
        private InputStream in;
        private int total;

        public Scan() {
            in = System.in;
        }

        public int scan() {
            if (total < 0)
                throw new InputMismatchException();
            if (index >= total) {
                index = 0;
                try {
                    total = in.read(buf);
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
                if (total <= 0)
                    return -1;
            }
            return buf[index++];
        }

        public long scanLong() {
            long integer = 0;
            int n = scan();
            while (isWhiteSpace(n))
                n = scan();
            int neg = 1;
            if (n == '-') {
                neg = -1;
                n = scan();
            }
            while (!isWhiteSpace(n)) {
                if (n >= '0' && n <= '9') {
                    integer *= 10;
                    integer += n - '0';
                    n = scan();
                } else
                    throw new InputMismatchException();
            }
            return neg * integer;
        }

        public double scanDouble() {
            double doub = 0;
            int n = scan();
            while (isWhiteSpace(n))
                n = scan();
            int neg = 1;
            if (n == '-') {
                neg = -1;
                n = scan();
            }
            while (!isWhiteSpace(n) && n != '.') {
                if (n >= '0' && n <= '9') {
                    doub *= 10;
                    doub += n - '0';
                    n = scan();
                } else
                    throw new InputMismatchException();
            }
            if (n == '.') {
                n = scan();
                double temp = 1;
                while (!isWhiteSpace(n)) {
                    if (n >= '0' && n <= '9') {
                        temp /= 10;
                        doub += (n - '0') * temp;
                        n = scan();
                    } else
                        throw new InputMismatchException();
                }
            }
            return doub * neg;
        }

        public String scanString() {
            StringBuilder sb = new StringBuilder();
            int n = scan();
            while (isWhiteSpace(n))
                n = scan();
            while (!isWhiteSpace(n)) {
                sb.append((char) n);
                n = scan();
            }
            return sb.toString();
        }

        public char scanChar() {
            int n = scan();
            while (isWhiteSpace(n))
                n = scan();
            return (char) n;
        }

        private boolean isWhiteSpace(int n) {
            if (n == ' ' || n == '\n' || n == '\r' || n == '\t' || n == -1)
                return true;
            return false;
        }
    }
}