#include <bits/stdc++.h>
using namespace std;
string d[1000];
string e[1000];
char g[1000];
string s;
int main() {
  map<char, int> mapc;
  map<string, int> maps;
  int a, b, c, l = 27, say = 0;
  cin >> a;
  char ch = 'a';
  for (int i = 1; i <= a; i++) cin >> d[i];
  for (int i = 1; i <= 26; i++) {
    g[i] = ch;
    e[i] = ch;
    ch++;
  }
  for (int i = 1; i <= 26; i++) {
    for (int j = i; j <= 26; j++) {
      e[l] = e[i] + e[j];
      l++;
    }
  }
  for (int i = 1; i <= a; i++) {
    for (int j = 0; j <= d[i].length() - 1; j++) {
      mapc[d[i][j]]++;
      mapc[d[i][j + 1]]++;
      s += d[i][j];
      s += d[i][j + 1];
      maps[s]++;
      s = "";
    }
  }
  for (int i = 1; i <= 26; i++) {
    if (mapc[g[i]] == 0) {
      cout << g[i] << endl;
      return 0;
    }
  }
  for (int i = 27; i <= l - 1; i++) {
    if (maps[e[i]] == 0) {
      cout << e[i] << endl;
      return 0;
    }
  }
  return 0;
}
