3
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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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 5
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 2
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 1
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 30
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 30
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 1
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 a
 e
 a
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 y
 b
 aa
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 ab
 z
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 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
void bubbleSort(int* arr, int size) {
  for (int i = 0; i < size; ++i)
    for (int j = i + 1; j < size; ++j)
      if (arr[i] > arr[j]) swap(arr[i], arr[j]);
}
void quickSort(int* arr, int l, int r) {
  int i = l, j = r;
  int x = arr[(l + r) / 2];
  while (i <= j) {
    while (arr[i] < x) i++;
    while (arr[j] > x) j--;
    if (i <= j) {
      swap(arr[i], arr[j]);
      i++;
      j--;
    }
  }
  if (j > l) quickSort(arr, l, j);
  if (r > i) quickSort(arr, i, r);
}
vector<string> names;
string solve(string s) {
  for (char ch = 'a'; ch <= 'z'; ++ch) {
    bool f = true;
    for (int i = 0; i < names.size() && f; ++i)
      if (names[i].find(s + ch, 0) != string::npos) f = false;
    if (f) return s + ch;
  }
  for (char ch = 'a'; ch <= 'z'; ++ch) return solve(s + ch);
}
int main() {
  int n;
  cin >> n;
  names.resize(n);
  for (int i = 0; i < n; ++i) cin >> names[i];
  string s = solve("");
  cout << s << endl;
  return 0;
}
