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 2
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 2
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 5
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 1
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 10
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 1
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#include <bits/stdc++.h>
using namespace std;
inline void wait(double seconds) {
  double endtime = clock() + (seconds * CLOCKS_PER_SEC);
  while (clock() < endtime) {
    ;
  }
}
inline int encodePos(int rpos, int cpos, int RR, int CC) {
  return rpos * CC + cpos;
}
inline int decodeR(int rpos, int RR, int CC) { return rpos / CC; }
inline int decodeC(int cpos, int RR, int CC) { return cpos % CC; }
template <class T>
inline T fastIn() {
  register char c = 0;
  register T a = 0;
  bool neg = false;
  while (c < 33) c = getchar();
  while (c > 33) {
    if (c == '-') {
      neg = true;
    } else {
      a = (a * 10) + (c - '0');
    }
    c = getchar();
  }
  return neg ? -a : a;
}
struct Node {
  int link[26];
  inline void set() { memset(link, 0, sizeof(link)); }
};
int const mxn = 1e4;
Node trie[mxn];
int cur_node;
inline void init_trie() {
  cur_node = 1;
  trie[0].set();
}
inline void read() {
  init_trie();
  int wordCount = fastIn<int>();
  while (wordCount--) {
    string buffer;
    cin >> buffer;
    int len = (int)(buffer).length();
    for (int i = 0; i < len; ++i) {
      int now = 0;
      for (int j = i; j < len; ++j) {
        int id = buffer[j] - 'a';
        if (!trie[now].link[id]) {
          trie[cur_node].set();
          trie[now].link[id] = cur_node++;
        }
        now = trie[now].link[id];
      }
    }
  }
}
struct qnode {
  int root;
  char ch;
  string res;
  qnode() { root = 0, res = ""; }
};
inline void proc() {
  queue<qnode> q;
  qnode tmp;
  for (char ch = 'a'; ch <= 'z'; ++ch) {
    tmp.root = 0;
    tmp.ch = ch;
    tmp.res = ch;
    q.push(tmp);
  }
  while (q.empty() == false) {
    qnode tmp2 = q.front();
    q.pop();
    if (!trie[tmp2.root].link[tmp2.ch - 'a']) {
      cout << tmp2.res << "\n";
      return;
    }
    int pointto = trie[tmp2.root].link[tmp2.ch - 'a'];
    for (char ch = 'a'; ch <= 'z'; ++ch) {
      tmp.root = pointto;
      tmp.ch = ch;
      tmp.res = tmp2.res + ch;
      q.push(tmp);
    }
  }
  cout << "damn!";
}
int main() {
  int kase = 1;
  for (int i = (0), _i = (kase); (i) < (_i); ++i) {
    read();
    proc();
  }
  return 0;
}
