#include <bits/stdc++.h>
using namespace std;
int n;
vector<string> vs;
string str;
bool non(string s) {
  for (int i = 0; i < int(vs.size()); ++i) {
    int ind = vs[i].find(s);
    if (ind >= 0) {
      return false;
    }
  }
  return true;
}
int main() {
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> str;
    vs.push_back(str);
  }
  for (int i = 0; i <= 26; ++i) {
    string si = "";
    if (i != 0) si = char(i + 'a' - 1);
    for (int j = 0; j <= 26; ++j) {
      string sj = si;
      if (j != 0) sj = si + char(j + 'a' - 1);
      for (int k = 0; k <= 26; ++k) {
        string sk = sj;
        if (k != 0) sk = sj + char(k + 'a' - 1);
        for (int z = 0; z <= 26; ++z) {
          string sz = sk;
          if (z != 0) sz = sk + char(z + 'a' - 1);
          for (int q = 0; q <= 26; ++q) {
            string sq = sz;
            if (q != 0) sq = sz + char(q + 'a' - 1);
            for (int w = 0; w <= 26; ++w) {
              string sw = sq;
              if (w != 0) sw = sq + char(w + 'a' - 1);
              if (sw == "") sw = "a";
              if (non(sw)) {
                cout << sw;
                return 0;
              }
            }
          }
        }
      }
    }
  }
  return 0;
}
