#include <bits/stdc++.h>
#pragma warning(disable : 4786)
#pragma comment(linker, "/STACK:16777216")
using namespace std;
struct Node {
  char name[26];
} N[35];
int n;
int flag[27];
int main() {
  int i, j, k, f;
  while (scanf("%d", &n) != EOF) {
    for (i = 0; i < n; i++) {
      scanf("%s", N[i].name);
      for (j = 0; N[i].name[j]; j++) flag[N[i].name[j] - 'a'] = 1;
    }
    f = 0;
    for (i = 0; i < 26; i++) {
      if (flag[i] == 0) {
        printf("%c\n", 'a' + i);
        f = 1;
        break;
      }
    }
    if (f == 0) {
      char c, d, res[45];
      int ff;
      for (c = 'a'; c <= 'z'; c++) {
        ff = 0;
        for (d = 'a'; d <= 'z'; d++) {
          res[0] = c;
          res[1] = d;
          res[2] = NULL;
          f = 0;
          for (i = 0; i < n; i++) {
            if (strstr(N[i].name, res)) {
              f = 1;
              break;
            }
          }
          if (f == 0) {
            ff = 1;
            break;
          }
        }
        if (ff == 1) break;
      }
      printf("%c%c\n", c, d);
    }
  }
  return 0;
}
