3
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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 2
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 1
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 30
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 1
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#include <bits/stdc++.h>
using namespace std;
map<string, bool> mymap;
int main() {
  int n, len;
  bool flag;
  char s[25];
  char ss[25];
  while (~scanf("%d", &n)) {
    mymap.clear();
    flag = 0;
    for (int i = 0; i < n; i++) {
      scanf("%s", s);
      len = strlen(s);
      for (int i = 0; i < len; i++) {
        ss[0] = s[i];
        ss[1] = 0;
        mymap[ss] = 1;
        if (i != len - 1) {
          ss[1] = s[i + 1];
          ss[2] = 0;
          mymap[ss] = 1;
        }
      }
    }
    for (int i = 0; i < 26; i++) {
      ss[0] = i + 'a';
      ss[1] = 0;
      if (!mymap[ss]) {
        flag = 1;
        printf("%s\n", ss);
        break;
      }
    }
    if (!flag) {
      for (int i = 0; i < 26; i++) {
        ss[0] = i + 'a';
        for (int j = 0; j < 26; j++) {
          ss[1] = j + 'a';
          ss[2] = 0;
          if (!mymap[ss]) {
            flag = 1;
            printf("%s\n", ss);
            break;
          }
        }
        if (flag) break;
      }
    }
  }
  return 0;
}
