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 30
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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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 2
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#include <bits/stdc++.h>
using namespace std;
const int N = 10000;
int sz, last, n;
string s, tmp;
struct automata {
  int len, link, next[28];
} st[N + N];
void insrt(int c) {
  int now = ++sz;
  st[now].len = st[last].len + 1;
  int p, q, cl;
  for (p = last; p != -1 && !st[p].next[c]; p = st[p].link) st[p].next[c] = now;
  if (p == -1)
    st[now].link = 0;
  else {
    q = st[p].next[c];
    if (st[p].len + 1 == st[q].len)
      st[now].link = q;
    else {
      cl = ++sz;
      st[cl].len = st[p].len + 1;
      st[cl].link = st[q].link;
      memcpy(st[cl].next, st[q].next, sizeof(st[cl].next));
      for (; p != -1 && st[p].next[c] == q; p = st[p].link) st[p].next[c] = cl;
      st[now].link = st[q].link = cl;
    }
  }
  last = now;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  while (n--) {
    cin >> tmp;
    tmp += "{";
    s += tmp;
  }
  st[0].link = -1;
  for (int i = 0; i < s.size(); i++) insrt(s[i] - 'a');
  string ans;
  int now = 0;
  while (true) {
    for (int c = 0; c < 26; c++) {
      if (!st[now].next[c]) {
        ans.push_back(char(c + 'a'));
        cout << ans << endl;
        return 0;
      }
    }
    ans.push_back('a');
    now = st[now].next[0];
  }
  return 0;
}
