3
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 1
z
 30
b
u
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k
p
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a
c
w
z
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t
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 5
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 2
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 5
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abcdefghijklmnopqrst
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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
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long int t;
  t = 1;
  while (t--) {
    long long int n, m, i, j, k;
    cin >> n;
    vector<string> v;
    while (n--) {
      string s;
      cin >> s;
      v.push_back(s);
    }
    string ans;
    char x = 'a';
    char y;
    long long int u = 0;
    for (x = 'a'; x <= 'z'; x++) {
      u = 0;
      for (i = 0; i < v.size(); i++) {
        int p = v[i].find(x);
        if (p != string::npos) {
          u = 1;
          break;
        }
      }
      if (!u) {
        cout << x;
        break;
      }
    }
    if (u) {
      u = 0;
      for (y = 'a'; y <= 'z'; y++) {
        u = 0;
        string temp1;
        temp1.push_back(y);
        for (x = 'a'; x <= 'z'; x++) {
          temp1.push_back(x);
          u = 0;
          for (i = 0; i < v.size(); i++) {
            int p = v[i].find(temp1);
            if (p != string::npos) {
              u = 1;
              break;
            }
          }
          if (!u) {
            cout << temp1;
            break;
          }
          if (!u) break;
          temp1.pop_back();
        }
        if (!u) break;
      }
    }
  }
  return 0;
}
