3
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 1
z
 30
b
u
p
n
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m
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j
k
p
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a
c
w
z
z
p
t
d
o
w
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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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abcdefghijklmnopqrst
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 2
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 1
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 30
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skrvnyz
avptgpjmenfzfxwckmt
 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

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