3
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 1
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 30
b
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p
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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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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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txwdjmohrrszorrnomc
 1
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 30
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avptgpjmenfzfxwckmt
 30
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 1
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a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
void next(string &s) {
  bool z = true;
  for (int i = 0; i < s.size(); i++)
    if (s[i] != 'z') z = false;
  if (z) {
    s.resize(s.size() + 1);
    for (int i = 0; i < s.size(); i++) s[i] = 'a';
  } else {
    for (int i = s.size() - 1; i >= 0; i--)
      if (s[i] == 'z')
        s[i] = 'a';
      else {
        s[i]++;
        break;
      }
  }
}
int main() {
  int n;
  scanf("%d", &n);
  string s[n];
  for (int i = 0; i < n; i++) {
    char c[21];
    scanf("%s", c);
    s[i] = c;
  }
  string t;
  bool ok;
  do {
    next(t);
    ok = true;
    for (int i = 0; i < n; i++) ok = ok && (s[i].find(t) == string::npos);
  } while (!ok);
  printf("%s", t.c_str());
}
