#include <bits/stdc++.h>
using namespace std;
int n, flag = 0;
int ch[27], ch2[27][27], ch3[27][27][27];
char s[40][40];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%s", s[i]);
    int tmp = strlen(s[i]);
    for (int j = 0; j < tmp; j++) ch[s[i][j] - 'a' + 1] = 1;
  }
  for (int i = 1; i <= 26; i++)
    if (!ch[i]) {
      printf("%c\n", 'a' + (i - 1));
      flag = 1;
      break;
    }
  if (!flag) {
    for (int i = 1; i <= n; i++) {
      int tmp = strlen(s[i]);
      for (int j = 1; j < tmp; j++)
        ch2[s[i][j - 1] - 'a' + 1][s[i][j] - 'a' + 1] = 1;
    }
    for (int i = 1; i <= 26; i++) {
      for (int j = 1; j <= 26; j++)
        if (!ch2[i][j]) {
          printf("%c%c\n", 'a' + (i - 1), 'a' + (j - 1));
          flag = 1;
          break;
        }
      if (flag == 1) break;
    }
  }
  if (!flag) {
    for (int i = 1; i <= n; i++) {
      int tmp = strlen(s[i]);
      for (int j = 2; j <= tmp; j++)
        ch3[s[i][j - 2] - 'a' + 1][s[i][j - 1] - 'a' + 1][s[i][j] - 'a' + 1] =
            1;
    }
    for (int i = 1; i <= 26; i++) {
      for (int j = 1; j <= 26; j++) {
        for (int k = 1; k <= 26; k++)
          if (!ch3[i][j][k]) {
            printf("%c%c%c\n", 'a' + (i - 1), 'a' + (j - 1), 'a' + (k - 1));
            flag = 1;
            break;
          }
        if (flag == 1) break;
      }
      if (flag == 1) break;
    }
  }
  return 0;
}
