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#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
int fx[] = {+1, -1, +0, +0, +1, +1, -1, -1, +0};
int fy[] = {+0, +0, +1, -1, +1, -1, +1, -1, +0};
template <typename T>
inline T GCD(T a, T b) {
  a = abs(a);
  b = abs(b);
  while (b) {
    a = a % b;
    swap(a, b);
  }
  return a;
}
template <typename T>
inline T LCM(T x, T y) {
  T tp = GCD(x, y);
  if ((x / tp) * 1. * y > 9e18) return 9e18;
  return (x / tp) * y;
}
template <typename T>
inline T BigMod(T A, T B, T M) {
  T ret = 1;
  while (B) {
    if (B & 1) ret = (ret * A) % M;
    A = (A * A) % M;
    B = B >> 1;
  }
  return ret;
}
template <typename T>
inline T ModInv(T A, T M) {
  return BigMod(A, M - 2, M);
}
template <typename T>
bool scani(T &n) {
  n = 0;
  bool got = false;
  bool negative = false;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') negative = true;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    got = true;
    n = n * 10 + c - 48;
    c = getchar();
  }
  if (negative) n = ~(n - 1);
  return got;
}
template <typename T>
void write(T n, bool line = true) {
  if (n < 0) {
    putchar('-');
    n = -n;
  }
  if (!n) {
    putchar('0');
    if (line) putchar('\n');
    return;
  }
  char buff[22];
  int len = 0;
  while (n) buff[len++] = n % 10 + 48, n /= 10;
  for (int i = len - 1; i >= 0; i--) putchar(buff[i]);
  if (line) putchar('\n');
}
int scans(char *a) {
  int i = 0;
  char c = 0;
  while (c < 33) c = getchar();
  while (c > 33) {
    a[i++] = c;
    c = getchar();
  }
  a[i] = 0;
  return i;
}
int main() {
  map<string, bool> mp;
  int n;
  scani(n);
  while (n--) {
    string s;
    cin >> s;
    int siz = s.size();
    for (int i = 0; i < siz; i++) {
      string tp = "";
      for (int j = i; j < siz; j++) {
        tp += s[j];
        mp[tp] = true;
      }
    }
  }
  string ans = "";
  do {
    int tp = 0;
    for (int i = 25; i >= 0; i--) {
      ans += i + 'a';
      if (!mp[ans]) {
        tp = i;
      }
      ans.pop_back();
    }
    ans += tp + 'a';
  } while (mp[ans]);
  cout << ans << endl;
  return 0;
}
