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 30
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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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#include <bits/stdc++.h>
using namespace std;
string s, q;
int N, i, j;
int a[27], nr, cr;
bool viz[105];
int main() {
  cin >> N;
  getline(cin, s);
  s.clear();
  for (i = 1; i <= N; ++i) {
    getline(cin, q);
    if (i != 1)
      s += q;
    else
      s = q;
  }
  int l = s.length();
  for (i = 0; i < l; ++i) {
    if (!viz[s[i] - 'a']) {
      viz[s[i] - 'a'] = 1;
      ++nr;
      if (nr == 26) {
        nr = 0;
        ++cr;
        for (i = 0; i < 26; ++i) a[i]--;
        memset(viz, 0, sizeof(viz));
      }
    }
    a[s[i] - 'a']++;
  }
  for (i = 1; i <= cr; ++i) cout << 'a';
  for (i = 0; i < 26; ++i)
    if (!a[i]) {
      cout << char(i + 'a');
      break;
    }
  return 0;
}
