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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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 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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 aa
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#include <bits/stdc++.h>
using namespace std;
map<string, int> mp;
vector<string> vp1;
vector<string> vp2;
bitset<1000> bs2;
bitset<1000> bs1;
void check_one(string ss) {
  for (int i = 0; i < ss.size(); i++) {
    string ds = "";
    ds += ss[i];
    int dd = find(vp1.begin(), vp1.end(), ds) - vp1.begin();
    bs1[dd] = 1;
  }
}
void check2(string ss) {
  for (int i = 0; i < ss.size(); i++) {
    for (int j = i + 1; j < ss.size(); j++) {
      string sd = "";
      sd += ss[i];
      sd += ss[j];
      int dd = find(vp2.begin(), vp2.end(), sd) - vp2.begin();
      bs2[dd] = 1;
    }
  }
}
int main() {
  bs1.reset();
  bs2.reset();
  for (char i = 'a'; i <= 'z'; i++) {
    string ss = "";
    ss += i;
    vp1.push_back(ss);
  }
  for (char i = 'a'; i <= 'z'; i++) {
    for (char j = 'a'; j <= 'z'; j++) {
      string ss = "";
      ss += i;
      ss += j;
      vp2.push_back(ss);
    }
  }
  sort(vp1.begin(), vp1.end());
  sort(vp2.begin(), vp2.end());
  string ss;
  int n;
  scanf("%d", &n);
  while (n--) {
    cin >> ss;
    check_one(ss);
    check2(ss);
  }
  for (int i = 0; i < vp1.size(); i++) {
    if (bs1[i] == 0) {
      cout << vp1[i] << endl;
      return 0;
    }
  }
  for (int i = 0; i < vp2.size(); i++) {
    if (bs2[i] == 0) {
      cout << vp2[i] << endl;
      return 0;
    }
  }
  return 0;
}
