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#include <bits/stdc++.h>
int top;
struct trie {
  int nxt[26];
  int flag;
  void rs() {
    flag = 0;
    memset(nxt, 0, sizeof(nxt));
  }
};
trie root[100000];
void init() {
  top = 1;
  root[0].rs();
}
void ins(char *str) {
  char *s = str;
  int p = 0;
  for (; *s; ++s) {
    int k = *s - 'a';
    if (!root[p].nxt[k]) {
      root[top].rs();
      root[p].nxt[k] = top++;
    }
    p = root[p].nxt[k];
  }
  root[p].flag = 1;
}
bool find(char *s) {
  int p = 0;
  while (*s) {
    int k = *s - 'a';
    if (root[p].nxt[k] == 0) return false;
    p = root[p].nxt[k];
    ++s;
  }
  return true;
}
int main(int argc, char *argv[]) {
  int n;
  char s[25];
  scanf("%d", &n);
  init();
  for (int i = 0; i < n; ++i) {
    scanf("%s", s);
    char *p = s;
    while (*p) {
      ins(p++);
    }
  }
  for (int i = 0; i < 26; ++i) {
    s[0] = 'a' + i;
    s[1] = '\0';
    if (!find(s)) {
      printf("%c\n", 'a' + i);
      return 0;
    }
  }
  for (int i = 0; i < 26; ++i)
    for (int j = 0; j < 26; ++j) {
      s[0] = 'a' + i;
      s[1] = 'a' + j;
      s[2] = '\0';
      if (!find(s)) {
        printf("%s\n", s);
        return 0;
      }
    }
  return EXIT_SUCCESS;
}
