3
rjnflsbpxqivrcdjptj
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 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
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o
a
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 5
splt
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 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
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ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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kopsdfgiopjipw
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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znligfxkgxzkcfeu
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doxnxbjwptnimjmsuijx
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
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skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
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cvdu
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jos
qy
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nayke
ge
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ubn
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wnjwjrf
t
bq
bw
 1
b

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

#include <bits/stdc++.h>
using namespace std;
template <typename A>
ostream &operator<<(ostream &cout, vector<A> const &v);
template <typename A, typename B>
ostream &operator<<(ostream &cout, pair<A, B> const &p) {
  return cout << "(" << p.first << ", " << p.second << ")";
}
template <typename A>
ostream &operator<<(ostream &cout, vector<A> const &v) {
  cout << "";
  for (int i = 0; i < v.size(); i++) {
    if (i) cout << " ";
    cout << v[i];
  }
  return cout << "\n";
}
template <typename A, typename B>
istream &operator>>(istream &cin, pair<A, B> &p) {
  cin >> p.first;
  return cin >> p.second;
}
template <typename A>
istream &operator>>(istream &cin, vector<A> &v) {
  for (auto &x : v) cin >> x;
  return cin;
}
long long min(long long a, int b) { return min(a, (long long)b); }
long long min(int a, long long b) { return min(b, (long long)a); }
long long max(long long a, int b) { return max(a, (long long)b); }
long long max(int a, long long b) { return max(b, (long long)a); }
void usaco(string filename) {
  freopen((filename + ".in").c_str(), "r", stdin);
  freopen((filename + ".out").c_str(), "w", stdout);
}
const long double pi = 3.14159265358979323846;
const long long mod = 1000000007;
const long long INF = 1e9 + 7;
long long n, m, k, q, l, r, x, y, z;
const long long sz = 30 + 1;
string second[sz], t;
long long ans = 0;
bool isSubstring(string &s1, string &s2) {
  int M = s1.length();
  int N = s2.length();
  for (int i = 0; i <= N - M; i++) {
    int j;
    for (j = 0; j < M; j++)
      if (s2[i + j] != s1[j]) break;
    if (j == M) return 1;
  }
  return 0;
}
void solve(int tc = 0) {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> second[i];
  }
  for (int i = 1; i <= n; i++) {
    string x(i, 'a');
    bool flag;
    while (true) {
      flag = 1;
      for (int i = 0; i < n; i++) {
        if (isSubstring(x, second[i])) {
          flag = 0;
          break;
        }
      }
      if (flag) break;
      while (x.back() == 'z') {
        x.pop_back();
      }
      if (x.empty()) break;
      x.back()++;
      while (x.size() < i) x.push_back('a');
    }
    if (flag) {
      cout << x << "\n";
      return;
    }
  }
}
int main() {
  int tc = 1;
  for (int t = 0; t < tc; t++) solve(t);
}
