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#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i;
  char a[40][30];
  scanf("%d", &n);
  for (i = 0; i < n; i++) scanf("%s", a[i]);
  int arr[26];
  int arr2[676];
  for (i = 0; i < 676; i++) {
    if (i < 26) arr[i] = 0;
    arr2[i] = 0;
  }
  int flag = 0;
  char temp = 'a';
  int j, k;
  for (j = 0; j < n; j++) {
    k = 0;
    while (a[j][k] != '\0') {
      arr[a[j][k] - 'a'] = 1;
      k++;
    }
  }
  for (i = 0; i < 26; i++) {
    if (arr[i] != 1) {
      flag = 1;
      break;
    }
  }
  if (flag == 1) {
    printf("%c\n", 'a' + i);
    return 0;
  }
  for (j = 0; j < n; j++) {
    k = 1;
    while (a[j][k] != '\0') {
      arr2[26 * (a[j][k - 1] - 'a') + a[j][k] - 'a'] = 1;
      k++;
    }
  }
  for (i = 0; i < 26 * 26; i++) {
    if (arr2[i] != 1) {
      flag = 1;
      break;
    }
  }
  if (flag == 1) {
    printf("%c%c\n", 'a' + i / 26, 'a' + i % 26);
    return 0;
  }
  int arr3[17576];
  for (i = 0; i < 26 * 26 * 26; i++) {
    arr3[i] = 0;
  }
  for (j = 0; j < n; j++) {
    k = 2;
    if (a[j][k - 1] == '\0') continue;
    while (a[j][k] != '\0') {
      arr3[26 * 26 * (a[j][k - 2] - 'a') + 26 * (a[j][k - 1] - 'a') + a[j][k] -
           'a'] = 1;
      k++;
    }
  }
  for (i = 0; i < 26 * 26 * 26; i++) {
    if (arr3[i] != 1) {
      flag = 1;
      break;
    }
  }
  if (flag == 1) {
    printf("%c%c%c\n", 'a' + i / (26 * 26), 'a' + i / 26, 'a' + i % 26);
    return 0;
  }
  return 0;
}
