3
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 1
z
 30
b
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n
f
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j
k
p
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a
c
w
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z
p
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d
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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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abcdefghijklmnopqrst
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 1
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 10
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 5
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abcdefghijklmnopqrst
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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
 a
 y
 b
 aa
 a
 ab
 z
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 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, j, k, i1, j1, k1, i2, j2, k2, q;
  char Name[30][21];
  bool t;
  cin >> n;
  for (i = 0; i < n; i++) cin >> Name[i];
  for (i = 0; i < 26; i++) {
    t = true;
    for (i1 = 0; i1 < n && t; i1++)
      for (j1 = 0; Name[i1][j1]; j1++)
        if ('a' + i == Name[i1][j1]) t = false;
    if (t) {
      cout << char('a' + i) << endl;
      return 0;
    }
  }
  for (i = 0; i < 26; i++)
    for (j = 0; j < 26; j++) {
      t = true;
      for (i1 = 0; i1 < n && t; i1++)
        for (j1 = 0; Name[i1][j1 + 1]; j1++)
          if ('a' + i == Name[i1][j1] && 'a' + j == Name[i1][j1 + 1]) t = false;
      if (t) {
        cout << char('a' + i) << char('a' + j) << endl;
        return 0;
      }
    }
  for (i = 0; i < 26; i++)
    for (j = 0; j < 26; j++)
      for (k = 0; k < 26; k++) {
        t = true;
        for (i1 = 0; i1 < n && t; i1++)
          for (j1 = 0; Name[i1][j1 + 2]; j1++)
            if ('a' + i == Name[i1][j1] && 'a' + j == Name[i1][j1 + 1] &&
                'a' + k == Name[i1][j1 + 2])
              t = false;
        if (t) {
          cout << char('a' + i) << char('a' + j) << endl;
          return 0;
        }
      }
  return 0;
}
