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#include <bits/stdc++.h>
using namespace std;
int tot;
char s[7000][22];
void hu(char *str, char ch);
bool cheak(char ss[]);
int main() {
  int k, n;
  while (scanf("%d", &n) != EOF) {
    tot = 0;
    memset(s, '\0', sizeof(s));
    for (int T = 1; T <= n; T++) {
      char str[22];
      scanf("%s", str);
      int len = strlen(str);
      for (int l = 1; l <= len; l++) {
        for (int i = 0; i < len - l + 1; i++) {
          int j = i + l - 1;
          tot++;
          for (k = i; k <= j; k++) hu(s[tot], str[k]);
        }
      }
    }
    bool ans = false;
    for (char z = 'a'; z <= 'z'; z++) {
      char ss[5];
      memset(ss, '\0', sizeof(ss));
      hu(ss, z);
      if (cheak(ss)) {
        printf("%s\n", ss);
        ans = true;
        break;
      }
    }
    if (!ans) {
      for (char y = 'a'; y <= 'z'; y++) {
        for (char z = 'a'; z <= 'z'; z++) {
          char ss[5];
          memset(ss, '\0', sizeof(ss));
          hu(ss, y);
          hu(ss, z);
          if (cheak(ss)) {
            printf("%s\n", ss);
            ans = true;
            break;
          }
        }
        if (ans) break;
      }
    }
    if (!ans) {
      for (char x = 'a'; x <= 'z'; x++) {
        for (char y = 'a'; y <= 'z'; y++) {
          for (char z = 'a'; z <= 'z'; z++) {
            char ss[5];
            memset(ss, '\0', sizeof(ss));
            hu(ss, x);
            hu(ss, y);
            hu(ss, z);
            if (cheak(ss)) {
              printf("%s\n", ss);
              ans = true;
              break;
            }
          }
          if (ans) break;
        }
        if (ans) break;
      }
    }
  }
}
void hu(char *str, char ch) {
  str[strlen(str)] = ch;
  str[strlen(str) + 1] = '\0';
  return;
}
bool cheak(char ss[]) {
  for (int i = 1; i <= tot; i++) {
    if (strcmp(ss, s[i]) == 0) {
      return false;
    }
  }
  return true;
}
