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
 e
 a
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 aa
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 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100000;
int n;
vector<char> tree;
vector<int> childs[MAXN];
queue<pair<int, string> > q;
void add(string s) {
  int ind = 0;
  for (int i = 0; i < s.length(); i++) {
    int next = -1;
    for (int j = 0; j < childs[ind].size(); j++) {
      if (tree[childs[ind][j]] == s[i]) next = childs[ind][j];
    }
    if (next == -1) {
      tree.push_back(s[i]);
      childs[ind].push_back(tree.size() - 1);
    }
    ind = next;
  }
}
bool comp(pair<int, string> p1, pair<int, string> p2) {
  return p1.second[p1.second.size() - 1] < p2.second[p2.second.size() - 1];
}
int main() {
  cin >> n;
  tree.push_back(' ');
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (int j = 0; j < s.length(); j++) add(s.substr(j));
  }
  q.push(pair<int, string>(0, ""));
  while (!q.empty()) {
    int size = q.size();
    for (int i = 0; i < size; i++) {
      pair<int, string> p = q.front();
      q.pop();
      bool vis[200];
      vector<pair<int, string> > v;
      for (char c = 'a'; c <= 'z'; c++) vis[c] = false;
      for (int j = 0; j < childs[p.first].size(); j++) {
        vis[tree[childs[p.first][j]]] = true;
        v.push_back(pair<int, string>(childs[p.first][j],
                                      p.second + tree[childs[p.first][j]]));
      }
      for (char c = 'a'; c <= 'z'; c++) {
        if (!vis[c]) {
          cout << p.second + c << endl;
          return 0;
        }
      }
      sort(v.begin(), v.end(), comp);
      for (int i = 0; i < v.size(); i++) q.push(v[i]);
    }
  }
}
