3
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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
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a
 a
 e
 a
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 y
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 aa
 a
 ab
 z
 e
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 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T mod_v(T num) {
  if (num >= 0)
    return num % 1000000007;
  else
    return (num % 1000000007 + 1000000007) % 1000000007;
}
template <class T>
inline T gcd(T a, T b) {
  a = abs(a);
  b = abs(b);
  while (b) b ^= a ^= b ^= a %= b;
  return a;
}
template <class T>
T fast_pow(T n, T p) {
  if (p == 0) return 1;
  if (p % 2) {
    T g = mod_v(mod_v(n) * mod_v(fast_pow(n, p - 1)));
    return g;
  } else {
    T g = fast_pow(n, p / 2);
    g = mod_v(mod_v(g) * mod_v(g));
    return g;
  }
}
template <class T>
inline T modInverse(T n) {
  return fast_pow(n, 1000000007 - 2);
}
template <class T>
inline void debug(string S1, T S2, string S3) {
  cout << S1 << S2 << S3;
}
bool equalTo(double a, double b) {
  if (fabs(a - b) <= 1e-9)
    return true;
  else
    return false;
}
bool notEqual(double a, double b) {
  if (fabs(a - b) > 1e-9)
    return true;
  else
    return false;
}
bool lessThan(double a, double b) {
  if (a + 1e-9 < b)
    return true;
  else
    return false;
}
bool lessThanEqual(double a, double b) {
  if (a < b + 1e-9)
    return true;
  else
    return false;
}
bool greaterThan(double a, double b) {
  if (a > b + 1e-9)
    return true;
  else
    return false;
}
bool greaterThanEqual(double a, double b) {
  if (a + 1e-9 > b)
    return true;
  else
    return false;
}
template <class T>
inline int in(register T& num) {
  register char c = 0;
  num = 0;
  bool n = false;
  while (c < 33) c = getchar();
  while (c > 33) {
    if (c == '-')
      n = true;
    else
      num = num * 10 + c - '0';
    c = getchar();
  }
  num = n ? -num : num;
  return 1;
}
vector<string> kp;
string s;
int n;
bool ok(string sp) {
  for (int i = 0; i < n; i++) {
    for (int j = 0; j <= ((int)kp[i].size()) - ((int)sp.size()); j++) {
      if (kp[i].substr(j, ((int)sp.size())) == sp) return false;
    }
  }
  return true;
}
bool cmp(string a, string b) {
  if (((int)a.size()) == ((int)b.size())) {
    for (int i = 0; i < ((int)a.size()); i++) {
      if (a[i] > b[i]) return false;
    }
    return true;
  }
  return ((int)a.size()) < ((int)b.size());
}
bool got = false;
string res;
void re(string fuck) {
  if (cmp(fuck, res) == false && ((int)fuck.size())) return;
  if (ok(fuck) && ((int)fuck.size())) {
    if (cmp(fuck, res)) res = fuck;
    return;
  }
  for (int i = 'a'; i <= 'z'; i++) {
    string x;
    x.push_back(i);
    re(fuck + x);
  }
}
int main() {
  in(n);
  for (int i = 0; i < n; i++) {
    cin >> s;
    kp.push_back(s);
  }
  res = "aaaaaaaaaaaaaaaaaaaaa";
  re("");
  cout << res;
  return 0;
}
