#include <bits/stdc++.h>
using namespace std;
char str[50][30];
char p[30];
char t[30];
int n;
int solve() {
  for (int i = 0; i < n; i++) {
    if (strstr(str[i], t) != NULL) {
      return 0;
    }
  }
  return 1;
}
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    scanf("%s", &str[i]);
  }
  p[0] = 'a' - 1;
  p[1] = 0;
  int len = 1;
  for (int i = 1;; i++) {
    p[0]++;
    int j = 0;
    while (p[j] == 'z' + 1) {
      p[j] = 'a';
      j++;
      if (p[j] == 0) {
        p[j] = 'a';
        p[j + 1] = 0;
        len = j + 1;
      } else {
        p[j]++;
      }
    }
    for (int k = 0; k < len; k++) {
      t[k] = p[len - k - 1];
    }
    t[len] = 0;
    if (solve() == 1) {
      break;
    }
  }
  printf("%s\n", t);
  return 0;
}
