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 30
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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;
int main(void) {
  int t, c[27] = {};
  string s[31];
  cin >> t;
  for (int i = 0; i < t; i++) {
    cin >> s[i];
    for (int j = 0; s[i][j] != '\0'; j++) c[s[i][j] - 96] = 1;
  }
  for (int i = 1; i < 27; i++) {
    if (c[i] == 0) {
      printf("%c", i + 96);
      return 0;
    }
  }
  int a[30];
  for (int i = 1; i < 27; i++) {
    int found;
    for (int j = 1, k = i; j <= k; j++, k--) {
      for (int m = 0; m < t; m++) {
        found = 0;
        int l = s[m].size();
        for (int n = 0; n < l - 1; n++) {
          if ((s[m][n] == j + 96 && s[m][n + 1] == k + 96)) {
            found = 1;
            break;
          }
        }
        if (found == 1) {
          for (int m = 0; m < t; m++) {
            found = 0;
            for (int n = 0; n < l - 1; n++) {
              if (s[m][n] == k + 96 && s[m][n + 1] == j + 96) {
                found = 1;
                break;
              }
            }
            if (found == 1)
              break;
            else
              continue;
          }
          if (found == 1)
            break;
          else if (found == 0) {
            printf("%c%c", k + 96, j + 96);
            return 0;
          }
        }
        if (found == 1)
          break;
        else if (found == 0) {
          continue;
        }
      }
      if (found == 0) {
        printf("%c%c", j + 96, k + 96);
        return 0;
      } else
        continue;
    }
  }
  return 0;
}
