#include <bits/stdc++.h>
using namespace std;
map<string, bool> substr;
int main() {
  int i, n, l, j;
  char strs[31][30], str[30];
  string test;
  scanf("%d", &n);
  while (n--) {
    scanf("%s", str);
    l = strlen(str);
    for (i = 0; i < l; i++) {
      for (j = 0; j <= i; j++) {
        strs[j][i - j] = str[i];
      }
    }
    for (i = 0; i < l; i++) {
      for (j = 0; j < (l - i); j++) {
        strs[i][l - i - j] = '\0';
        test.assign(strs[i]);
        substr[test] = 1;
      }
    }
  }
  substr.size();
  char startstr[10000];
  startstr[0] = '\0';
  int pos = -1;
  l = 0;
  char ch[2];
  ch[0] = 'a';
  ch[1] = '\0';
  while (1) {
    if (pos == -1) {
      l += 1;
      strcat(startstr, ch);
      pos = l - 1;
    } else if (startstr[pos] == 'z') {
      while (pos != -1 && startstr[pos] == 'z') {
        startstr[pos] = 'a';
        pos -= 1;
      }
      if (pos == -1) {
        l += 1;
        strcat(startstr, ch);
        pos = l - 1;
      } else {
        startstr[pos] += 1;
        pos = l - 1;
      }
    } else {
      startstr[pos] += 1;
      test.assign(startstr);
    }
    test.assign(startstr);
    if (substr.find(test) == substr.end()) {
      cout << test << endl;
      return 0;
      break;
    }
  }
  return 0;
}
