3
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 1
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 30
b
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k
p
h
a
c
w
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z
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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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 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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 l
 a

#include <bits/stdc++.h>
using namespace std;
bool lucky(int n) {
  while (n != 0) {
    if (n % 10 != 4 && n % 10 != 7) return 0;
    n /= 10;
  }
  return 1;
}
int gcd(int a, int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int main() {
  string s[30], ans = "";
  int n, m = 0, k = 0, p = 0;
  bool ch[26] = {0}, jh[26][26] = {0};
  cin >> n;
  int i, j;
  for (i = 0; i < n; i++) {
    cin >> s[i];
    for (j = 0; j < s[i].size(); j++) ch[int(s[i][j] - 97)] = 1;
    for (j = 0; j < s[i].size() - 1; j++) {
      jh[int(s[i][j] - 97)][int(s[i][j + 1] - 97)] = 1;
    }
  }
  for (i = 0; i < 26; i++) {
    if (!ch[i]) {
      ans += char(i + 97);
      break;
    }
  }
  if (ans != "") return cout << ans << endl, 0;
  for (i = 0; i < 26; i++) {
    for (j = 0; j < 26; j++)
      if (!jh[i][j]) break;
    if (!jh[i][j]) break;
  }
  if (!jh[i][j]) cout << char(i + 97) << "" << char(j + 97) << endl;
}
