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#include <bits/stdc++.h>
using namespace std;
long long int Pow(long long int B, long long int P) {
  long long int R = 1;
  while (P > 0) {
    if (P % 2 == 1) R = (R * B);
    P /= 2;
    B = (B * B);
  }
  return R;
}
int BigMod(long long int B, long long int P, long long int M) {
  long long int R = 1;
  while (P > 0) {
    if (P % 2 == 1) {
      R = (R * B) % M;
    }
    P /= 2;
    B = (B * B) % M;
  }
  return (int)R;
}
int prime[90000];
bool flag[1010];
int sieve() {
  int i, j, total = 0, val;
  val = sqrt(1010) + 1;
  flag[0] = flag[1] = 1;
  for (i = 2; i < val; i++)
    if (!flag[i])
      for (j = i; j * i <= 1010; j++) flag[i * j] = 1;
  for (i = 2; i <= 1010; i++)
    if (!flag[i]) prime[total++] = i;
  return total;
}
template <class T>
inline T lcm(T a, T b) {
  if (a < 0) return lcm(-a, b);
  if (b < 0) return lcm(a, -b);
  return a * (b / (gcd(a, b)));
}
template <class T>
inline T gcd(T a, T b) {
  if (a < 0) return gcd(-a, b);
  if (b < 0) return gcd(a, -b);
  return (b == 0) ? a : gcd(b, a % b);
}
int main() {
  int i, j, n;
  string str[1010];
  int color[1010];
  cin >> n;
  for (i = 0; i < n; i++) cin >> str[i];
  for (i = 0; i < n; i++) {
    if (!color[str[i][0]]) color[str[i][0]] = 1;
    for (j = 1; j < (int)str[i].size(); j++) {
      if (!color[str[i][j]]) {
        color[str[i][j]] = 1;
      } else {
        if (str[i][j - 1] == str[i][j]) {
          int a = color[str[i][j - 1]];
          color[str[i][j]] = ++a;
        } else
          continue;
      }
    }
  }
  bool mark = false;
  vector<char> res;
  for (i = 1; i <= 20; i++) {
    for (j = 0; j < 26; j++) {
      char ch = 'a' + j;
      if (!color[ch]) {
        res.push_back(ch);
        mark = true;
        break;
      } else {
        color[ch] = color[ch] - 1;
      }
    }
    if (mark)
      break;
    else
      res.push_back('a');
  }
  for (i = 0; i < (int)res.size(); i++) cout << res[i];
  cout << endl;
  return 0;
}
