3
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 1
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 30
b
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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
 a
 e
 a
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 aa
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 ab
 z
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#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  vector<string> nam;
  for (int i = 0; i < n; i++) {
    string x;
    cin >> x;
    nam.push_back(x);
  }
  vector<string> sho;
  sho.push_back("a");
  for (int i = 1; i < 26; i++) {
    string hh = "";
    hh += (char)('a' + i);
    sho.push_back(hh);
  }
  int in = 0;
  while (1) {
    bool t = 1;
    for (int i = 0; i < n; i++) {
      if (nam[i].find(sho[in]) != -1) {
        t = 0;
        break;
      }
    }
    for (int i = 0; i < 26; i++) {
      string hh = sho[in];
      hh += (char)('a' + i);
      sho.push_back(hh);
    }
    if (t) {
      cout << sho[in];
      break;
    }
    in++;
  }
}
