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#include <bits/stdc++.h>
#pragma comment(linker, "/stack:640000000")
using namespace std;
const double EPS = 1e-9;
const int INF = 0x7f7f7f7f;
const double PI = acos(-1.0);
template <class T>
inline T _abs(T n) {
  return ((n) < 0 ? -(n) : (n));
}
template <class T>
inline T _max(T a, T b) {
  return (!((a) < (b)) ? (a) : (b));
}
template <class T>
inline T _min(T a, T b) {
  return (((a) < (b)) ? (a) : (b));
}
template <class T>
inline T _swap(T& a, T& b) {
  a = a ^ b;
  b = a ^ b;
  a = a ^ b;
}
template <class T>
inline T gcd(T a, T b) {
  return (b) == 0 ? (a) : gcd((b), ((a) % (b)));
}
template <class T>
inline T lcm(T a, T b) {
  return ((a) / gcd((a), (b)) * (b));
}
template <typename T>
string NumberToString(T Number) {
  ostringstream second;
  second << Number;
  return second.str();
}
struct debugger {
  template <typename T>
  debugger& operator,(const T& v) {
    cerr << v << " ";
    return *this;
  }
} dbg;
vector<string> allpossible;
void call(int pos, int len, string str) {
  if (len == 0) {
    allpossible.push_back(str);
    return;
  }
  if (pos == 26) {
    return;
  }
  call(pos, len - 1, str + (char)(pos + 97));
  call(pos + 1, len, str);
}
int main() {
  for (int i = 0; i < 26; i++) {
    string a = "";
    a += (char)(i + 97);
    allpossible.push_back(a);
  }
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < 26; j++) {
      string a = "";
      a += (char)(i + 97);
      a += (char)(j + 97);
      allpossible.push_back(a);
    }
  };
  int n;
  while (cin >> n) {
    ;
    string a[n + 1];
    for (int i = 0; i < n; i++) {
      cin >> a[i];
    }
    int flag = 0;
    int now = 0;
    while (!flag) {
      string b = allpossible[now];
      int f = 0;
      for (int i = 0; i < n; i++) {
        int koyta = 0;
        for (int j = 0, k = 0; j < a[i].size(); j++) {
          if (b[k] == a[i][j]) {
            koyta++;
            k++;
          }
        }
        if (koyta >= b.size()) {
          f = 1;
          now++;
          break;
        }
      }
      if (f == 0) {
        flag = 1;
        break;
      }
    }
    cout << allpossible[now] << endl;
  }
  return 0;
}
