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#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const double eps = 1e-5;
const int maxn = 100 + 10;
const int inf = 0x3f3f3f3f;
const long long linf = 0x3f3f3f3f3f3f3f3f;
const double PI = acos(-1);
const int Times = 11;
long long qpow_mod(long long a, long long b, long long c) {
  long long ans = 1;
  while (b) {
    if (b & 1) {
      ans *= a;
      ans %= c;
    }
    a *= a;
    a %= c;
    b >>= 1;
  }
  return ans;
}
inline int read() {
  int ret = 0, sgn = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') sgn = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    ret = ret * 10 + ch - '0';
    ch = getchar();
  }
  return ret * sgn;
}
int cnt[28];
string t;
set<string> s;
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    cin >> t;
    int d = t.size();
    for (int j = 0; j < d; j++) {
      int tmp = t[j] - 'a';
      cnt[tmp]++;
      if (j == d - 1) break;
      string a = t.substr(j, 2);
      s.insert(a);
    }
  }
  int pos = -1;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] == 0) {
      pos = i;
      break;
    }
  }
  if (pos != -1) {
    printf("%c\n", pos + 'a');
    return 0;
  }
  int flag = 0;
  char b[2];
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < 26; j++) {
      b[0] = char(i + 'a');
      b[1] = char(j + 'a');
      string bs = b;
      bs = bs.substr(0, 2);
      if (s.find(bs) == s.end()) {
        flag = 1;
        cout << bs << endl;
        break;
      }
    }
    if (flag) break;
  }
  return 0;
}
