#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:16777216")
using namespace std;
const double PI = 2.0 * acos(0.0);
const int MAXN = 33;
int n, m;
string s;
bool d1[26];
bool d2[700];
int encode(char x, char y) { return (x - 'a') * 26 + (y - 'a'); }
int main() {
  memset(d1, false, sizeof(d1));
  memset(d2, false, sizeof(d2));
  cin >> n;
  for (int i = 0, _n = (n); i < _n; i++) {
    cin >> s;
    m = s.length();
    for (int j = 0, _n = (m); j < _n; j++) {
      d1[s[j] - 'a'] = true;
      if (j + 1 < m) d2[encode(s[j], s[j + 1])] = true;
    }
  }
  char r1, r2;
  int x;
  for (int i = 0, _n = (26); i < _n; i++) {
    if (!d1[i]) {
      r1 = 'a' + i;
      cout << r1 << endl;
      return 0;
    }
  }
  for (int i = 0, _n = (700); i < _n; i++) {
    if (!d2[i]) {
      r1 = 'a' + (i / 26);
      r2 = 'a' + (i % 26);
      cout << r1 << r2 << endl;
      return 0;
    }
  }
}
