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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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 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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#include <bits/stdc++.h>
const long long INF = 1000000000;
using namespace std;
char s[22];
int n;
map<string, int> q;
struct node {
  string s;
};
string bfs() {
  queue<node> p;
  node x;
  x.s = "";
  p.push(x);
  while (!q.empty()) {
    x = p.front();
    p.pop();
    int i;
    for (i = 0; i < 26; i++) {
      node t = x;
      t.s += 'a' + i;
      if (q[t.s] == 0) return t.s;
      p.push(t);
    }
  }
}
int main() {
  while (scanf("%d", &n) != EOF) {
    q.clear();
    int i, j, k;
    for (i = 0; i < n; i++) {
      scanf("%s", s);
      int l = strlen(s);
      for (j = 0; j < l; j++) {
        string str = "";
        str += s[j];
        q[str] = 1;
        for (k = j + 1; k < l; k++) {
          str += s[k];
          q[str] = 1;
        }
      }
    }
    cout << bfs() << endl;
  }
  return 0;
}
