3
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 1
z
 30
b
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f
m
t
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j
k
p
h
a
c
w
z
z
p
t
d
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w
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q
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y
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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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 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
 e
 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
vector<string> vec;
void init() {
  vec.push_back("a");
  vec.push_back("b");
  vec.push_back("c");
  vec.push_back("d");
  vec.push_back("e");
  vec.push_back("f");
  vec.push_back("g");
  vec.push_back("h");
  vec.push_back("i");
  vec.push_back("j");
  vec.push_back("k");
  vec.push_back("l");
  vec.push_back("m");
  vec.push_back("n");
  vec.push_back("o");
  vec.push_back("p");
  vec.push_back("q");
  vec.push_back("r");
  vec.push_back("s");
  vec.push_back("t");
  vec.push_back("u");
  vec.push_back("v");
  vec.push_back("w");
  vec.push_back("x");
  vec.push_back("y");
  vec.push_back("z");
}
string intToStr(long long n) {
  int mod;
  string s = "";
  while (n > 26) {
    mod = n % 26;
    s = s + vec[mod];
    n /= 26;
  }
  n--;
  s = vec[n] + s;
  return s;
}
int main() {
  init();
  int N;
  char cad[25];
  string s, ns;
  set<string> mapa;
  while (scanf("%d\n", &N) != EOF) {
    mapa.clear();
    for (int i = 0; i < N; i++) {
      scanf("%s\n", cad);
      s = string(cad);
      for (int i = 0; i < (int)s.size(); i++)
        for (int j = 1; j <= (int)s.size(); j++) {
          ns = s.substr(i, j);
          if (mapa.find(ns) == mapa.end()) mapa.insert(ns);
        }
    }
    long long cont = 1;
    bool terminado = false;
    while (!terminado) {
      ns = intToStr(cont);
      if (mapa.find(ns) == mapa.end()) {
        printf("%s\n", ns.c_str());
        terminado = true;
      } else
        cont++;
    }
  }
  return 0;
}
