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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 1
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#include <bits/stdc++.h>
using namespace std;
string s, a, ans;
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> s;
    a.append("0");
    a.append(s);
  }
  int l = a.length();
  int c;
  for (int i = 'a'; i <= 'z'; i++) {
    c = 1;
    for (int j = 0; j < l; j++) {
      if (a[j] == i) {
        c = 0;
        break;
      }
    }
    if (c) {
      a[999] = i;
      cout << a[999];
      return 0;
    }
  }
  for (int i = 'a'; i <= 'z'; i++)
    for (int k = 'a'; k <= 'z'; k++) {
      c = 1;
      for (int j = 0; j < l - 1; j++) {
        if (a[j] == i && a[j + 1] == k) {
          c = 0;
          break;
        }
      }
      if (c) {
        a[998] = i;
        a[999] = k;
        cout << a[998] << a[999];
        return 0;
      }
    }
  for (int i = 'a'; i <= 'z'; i++)
    for (int m = 'a'; m <= 'z'; m++)
      for (int k = 'a'; k <= 'z'; k++) {
        c = 1;
        for (int j = 0; j < l - 2; j++) {
          if (a[j] == i && a[j + 1] == m && a[j + 2] == k) {
            c = 0;
            break;
          }
        }
        if (c) {
          a[997] = i;
          a[998] = m;
          a[999] = k;
          cout << a[997] << a[998] << a[999];
          return 0;
        }
      }
  return 0;
}
