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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 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
 a
 e
 a
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 y
 b
 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
vector<string> v, temp;
int main() {
  int n, m, i, j, k;
  string x, y;
  scanf("%d", &n);
  for (i = 1; i <= n; i++) {
    cin >> x;
    m = x.length();
    y = "";
    for (j = 0; j < m; j++) {
      y = "";
      for (k = j; k < m; k++) {
        y += x[j];
        v.push_back(y);
      }
    }
  }
  for (i = 0; i < 26; i++) {
    string s(1, char(i + 97));
    temp.push_back(s);
  }
  for (i = 0; i < 26; i++) {
    y = temp[i];
    for (j = 0; j < 26; j++) {
      y = temp[i] + temp[j];
      temp.push_back(y);
    }
  }
  bool bb = true;
  for (auto j : temp) {
    bb = 1;
    for (auto i : v) {
      if (j == i) {
        bb = 0;
        break;
      }
    }
    if (bb) {
      cout << j << endl;
      ;
      return 0;
    }
  }
}
