#include <bits/stdc++.h>
using namespace std;
string a[33];
void add1(char s[]) {
  int len = strlen(s) - 1;
  while (s[len] + 1 > 'z') len--;
  if (len >= 0) {
    s[len]++;
    for (int i = len + 1; s[i] != '\0'; i++) s[i] = 'a';
  } else {
    len = strlen(s);
    for (int i = 0; i <= len; i++) s[i] = 'a';
    s[len + 1] = '\0';
  }
}
int main() {
  int t;
  char res[30];
  res[0] = 'a', res[1] = '\0';
  scanf("%d", &t);
  for (int i = 0; i < t; i++) cin >> a[i];
  int fl = 0;
judge:
  fl = 0;
  for (int i = 0; i < t; i++) {
    if (a[i].find(res) != string::npos) {
      fl = 1;
      break;
    }
  }
  if (fl) {
    add1(res);
    goto judge;
  } else
    puts(res);
}
