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
 a
 y
 b
 aa
 a
 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
inline void inpint(int &n) {
  n = 0;
  register int ch = getchar_unlocked();
  bool sign = 0;
  while (ch < 48 || ch > 57) {
    if (ch == '-') sign = 1;
    ch = getchar_unlocked();
  }
  while (ch >= 48 && ch <= 57) {
    n = (n << 3) + (n << 1) + ch - 48, ch = getchar_unlocked();
  }
  if (sign) n = -n;
}
inline int sqr(int x) { return x * x; }
inline int cube(int x) { return x * x * x; }
inline long long sqrLL(long long x) { return x * x; }
inline long long cubeLL(long long x) { return x * x * x; }
const long long LLINF = 9223372036854775807LL;
const long long LLINF17 = 100000000000000000LL;
const int INF = 2147483647;
const int INF9 = 1000000000;
const int MOD = 1000000007;
const double eps = 1e-7;
const double PI = acos(-1.0);
int dr[] = {1, 0, -1, 0, -1, 1, 1, -1};
int dc[] = {0, -1, 0, 1, 1, 1, -1, -1};
int n;
vector<string> v;
inline bool check(string cur) {
  for (int(i) = (0); (i) < (n); (i)++) {
    if (v[i].find(cur) != string::npos) {
      return 0;
    }
  }
  return 1;
}
int main() {
  string a = "ab", b = "b";
  cin >> n;
  for (int(i) = (1); (i) <= (n); (i)++) {
    string s;
    cin >> s;
    v.push_back(s);
  }
  string ans = "zz";
  for (int(i) = (0); (i) < (26); (i)++) {
    string cur = "";
    cur.push_back((char)(i + 'a'));
    if (check(cur)) {
      ans = cur;
      break;
    }
    for (int(j) = (0); (j) < (26); (j)++) {
      cur.push_back((char)(j + 'a'));
      if (check(cur)) {
        ans = cur;
        goto end;
      }
      cur.erase(cur.end() - 1);
    }
  }
end:
  cout << ans << endl;
  return 0;
}
