3
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 1
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 30
b
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 5
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 2
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 5
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abcdefghijklmnopqrst
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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;
vector<string> w;
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    w.push_back(s);
  }
  for (char i = 'a'; i <= 'z'; i++) {
    int q = 0;
    bool a = false;
    for (int j = 0; j < w.size(); j++) {
      a = false;
      for (int k = 0; k < w[j].size(); k++)
        if (w[j][k] == i) a = true;
      if (not a) q++;
    }
    if (q == n) {
      cout << i;
      return 0;
    }
  }
  for (char a = 'a'; a <= 'z'; a++)
    for (char b = 'a'; b <= 'z'; b++) {
      bool t = false;
      int q = 0;
      for (int k = 0; k < w.size(); k++) {
        t = false;
        for (int i = 0; i < w[k].size(); i++)
          if (i + 1 < n && w[k][i] == a && w[k][i + 1] == b) t = true;
        if (not t) q++;
      }
      if (q == n) {
        cout << a << b;
        return 0;
      }
    }
  return 0;
}
