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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 y
 b
 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 1000;
const int inf = 1e6;
void read(int &x) {
  char ch;
  bool flag = 0;
  for (ch = getchar(); !isdigit(ch) && ((flag |= (ch == '-')) || 1);
       ch = getchar())
    ;
  for (x = 0; isdigit(ch); x = (x << 1) + (x << 3) + ch - 48, ch = getchar())
    ;
  x *= 1 - 2 * flag;
}
string s[maxn];
int res[maxn];
int main() {
  int n;
  read(n);
  memset(res, 0, sizeof(res));
  for (int i = 0; i < n; i++) {
    cin >> s[i];
    int siz = s[i].size();
    for (int j = 0; j < siz; j++) {
      res[s[i][j] - 'a']++;
    }
  }
  sort(s, s + n);
  for (int i = 0; i < 'z' - 'a'; i++) {
    if (!res[i]) {
      return 0 * printf("%c", (i + 'a'));
    }
  }
  for (int i = 0; i < s[0].size(); i++) {
    printf("%c", i + 'a');
  }
  return 0;
}
