#include <bits/stdc++.h>
int dr[] = {2, 2, -2, -2, 1, -1, 1, -1};
int dc[] = {1, -1, 1, -1, 2, 2, -2, -2};
int dr1[] = {0, 0, 0, 1, 1, 1, -1, -1, -1};
int dc1[] = {-1, 0, 1, -1, 0, 1, -1, 0, 1};
int dr2[] = {0, 0, 1, -1};
int dc2[] = {1, -1, 0, 0};
using namespace std;
long long int t[100005][26];
long long int dp[22][100005];
string ans;
long long int cal(long long int x, long long int now) {
  if (x == 0) return (now == 0);
  long long int &r = dp[x][now];
  if (r != -1) return r;
  r = 0;
  for (long long int i = 0; i < 26; i++) {
    long long int p = t[now][i];
    r |= cal(x - 1, p);
    if (r == 1) {
      ans.push_back(i + 'a');
      return r;
    }
  }
  return r;
}
int main() {
  long long int n, i, j1, j;
  string t1;
  while (cin >> n) {
    long long int node = 1;
    for (i = 1; i <= n; i++) {
      cin >> t1;
      for (j = 0; j < t1.size(); j++) {
        long long int now = 1;
        for (j1 = j; j1 < t1.size(); j1++) {
          long long int p = t1[j1] - 'a';
          if (t[now][p] == 0) t[now][p] = ++node;
          now = t[now][p];
        }
      }
    }
    memset(dp, -1, sizeof(dp));
    for (i = 1; i <= 21; i++) {
      long long int p = cal(i, 1);
      if (p == 1) break;
    }
    reverse(ans.begin(), ans.end());
    cout << ans << endl;
  }
  return 0;
}
