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 30
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 2
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 5
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#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, flag1, flag2, i, j, m, n, o, p, k;
  int ab[26] = {0};
  char str[1];
  cin >> c;
  char a[c][22];
  for (i = 0; i < c; i++) cin >> a[i];
  for (i = 0; i < c; i++) {
    p = strlen(a[i]);
    for (j = 0; j < p; j++) {
      m = a[i][j] - 97;
      ab[m]++;
    }
  }
  for (i = 0; i < 26; i++) {
    if (ab[i] == 0) {
      cout << char(97 + i);
      flag1 = 1;
      break;
    }
  }
  if (flag1 != 1) {
    int x = 0;
    for (i = 0; i < 26; i++) {
      for (j = 0; j < 26; j++) {
        str[0] = i + 97;
        str[1] = j + 97;
        x = 0;
        for (k = 0; k < c; k++) {
          char* ch3;
          ch3 = strstr(a[k], str);
          if (ch3 != NULL) {
            x = 1;
            break;
          }
        }
        if (x == 0) break;
      }
      if (x == 0) break;
    }
    cout << str;
  }
}
