#include <bits/stdc++.h>
using namespace std;
template <class T>
void inline maxi(T &a, const T &b) {
  a = max(a, b);
}
template <class T>
void inline mini(T &a, const T &b) {
  a = min(a, b);
}
void shownum(int n, int m, int W = 6) {
  for (int i = n; i <= m; ++i) {
    cout << setw(W) << i;
  }
  cout << endl;
}
template <class T>
void show(T &a, int n, int m, int W = 6) {
  for (int i = n; i <= m; ++i) {
    cout << setw(W) << a[i];
  }
  cout << endl;
}
template <class T>
void show(T &a, int W = 6) {
  for (int(u) = 0; (u) < int(a.size()); ++u) {
    cout << setw(W) << a[u];
  }
  cout << endl;
}
const int maxnum = int(400 * 30) + 10;
const int maxint = INT_MAX;
const int minint = INT_MIN;
char t[100];
int ord(char x) { return x - 'a'; }
int tree[maxnum][26];
bool find(string t) {
  int u = 0;
  for (int(i) = 0; (i) < int(t.length()); ++i) {
    int c = ord(t[i]);
    if (!tree[u][c])
      return false;
    else
      u = tree[u][c];
  }
  return true;
}
void mmain() {
  int n;
  scanf("%d", &n);
  memset(tree, 0, sizeof(tree));
  int len = 1;
  for (int(i) = 0; (i) < int(n); ++i) {
    scanf("%s", t);
    int l = strlen(t);
    for (int(j) = 0; (j) < int(l); ++j) {
      int u = 0;
      for (int k = j; k < l; ++k) {
        int c = ord(t[k]);
        if (!tree[u][c]) {
          tree[u][c] = len++;
        }
        u = tree[u][c];
      }
    }
  }
  queue<string> q;
  for (int(i) = 0; (i) < int(26); ++i) q.push(string() + char('a' + i));
  while (!q.empty()) {
    string node = q.front();
    q.pop();
    if (!find(node)) {
      cout << node << endl;
      break;
    }
    for (int(i) = 0; (i) < int(26); ++i) q.push(node + char('a' + i));
  }
}
int main() { mmain(); }
