3
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 1
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 30
b
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a
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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
b

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

#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-9;
const long long INF = 1LL << 60;
const double PI = acos(-1.0);
int nextint() {
  int t;
  if (scanf("%d", &t) != 1) throw "";
  return t;
}
double nextdouble() {
  double t;
  if (scanf("%lf", &t) != 1) throw "";
  return t;
}
char _[16];
char nextchar() {
  if (scanf("%s", _) != 1) throw "";
  return _[0];
}
string nextstring() {
  string s;
  cin >> s;
  return s;
}
int main() {
  vector<string> v(nextint());
  for (int i = 0; i < v.size(); i++) v[i] = nextstring();
  for (char ci = 'a'; ci <= 'z'; ci++) {
    bool ok = true;
    for (int i = 0; i < v.size(); i++)
      if (v[i].find(ci) != string::npos) ok = false;
    if (ok) {
      cout << ci << endl;
      return 0;
    }
  }
  for (char ci = 'a'; ci <= 'z'; ci++) {
    for (char cj = 'a'; cj <= 'z'; cj++) {
      bool ok = true;
      for (int i = 0; i < v.size(); i++)
        if (v[i].find(string("" + ci + cj)) != string::npos) ok = false;
      if (ok) {
        cout << string("" + ci + cj) << endl;
        return 0;
      }
    }
  }
  return 0;
}
