#include <bits/stdc++.h>
using namespace std;
char s[31][21];
char c[22];
int n;
int check() {
  for (int i = 0; i < n; i++) {
    for (int j = 0; s[i][j] != '\0'; j++) {
      int k = 0;
      for (; s[i][j + k] != '\0' && c[k] != '\0' && s[i][j + k] == c[k]; k++)
        ;
      if (c[k] == '\0') {
        return 0;
      }
    }
  }
  return 1;
}
void recurse(int p, int d) {
  if (p == d) {
    int k = check();
    if (k == 1) {
      cout << c;
      exit(0);
    }
    return;
  }
  for (int i = 0; i < 26; i++) {
    c[p] = 'a' + i;
    c[p + 1] = '\0';
    recurse(p + 1, d);
  }
}
void generate() {
  for (int i = 1; i < 20; i++) {
    recurse(0, i);
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> s[i];
  }
  generate();
  return 0;
};
