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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 aa
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#include <bits/stdc++.h>
using namespace std;
int N;
int A[101];
int T, TS;
string S[31];
set<string> Q;
string Ans;
bool Find(int i, int M) {
  if (i > M) {
    set<string>::iterator x;
    x = Q.find(Ans);
    if (x == Q.end()) return true;
    return false;
  }
  for (int j = 0; j < 26; j++) {
    char a;
    a = j + 'a';
    Ans = Ans + a;
    if (Find(i + 1, M)) return true;
    int tp;
    tp = Ans.length();
    Ans = Ans.substr(0, tp - 1);
  }
  return false;
}
void check() {
  for (int i = 1; i <= 21; i++) {
    if (Find(1, i)) {
      cout << Ans << endl;
      break;
    }
  }
}
int main() {
  scanf("%d", &N);
  for (int i = 1; i <= N; i++) cin >> S[i];
  TS = 0;
  Ans = "";
  for (int i = 1; i <= N; i++) {
    int L;
    L = S[i].length();
    for (int j = 0; j < L; j++) {
      string St;
      St = "";
      for (int k = j; k < L; k++) {
        St += S[i][k];
        Q.insert(St);
      }
    }
  }
  check();
  return 0;
}
