#include <bits/stdc++.h>
bool mark[26];
using namespace std;
vector<string> names;
int main() {
  int n;
  scanf("%d", &n);
  char title[21];
  int count = 0;
  for (int i = 0; i < n; i++) {
    scanf("%s", title);
    names.push_back(title);
    for (int j = 0; j < title[j] != '\0'; j++) {
      int index = title[j] - 'a';
      if (!mark[index]) {
        mark[index] = 1;
        count++;
      }
    }
  }
  if (count < 26) {
    for (int i = 0; i < 26; i++) {
      if (!mark[i]) {
        printf("%c", 'a' + i);
        break;
      }
    }
  } else {
    string ckStr;
    for (int i = 'a'; i <= 'z'; ++i) {
      ckStr = i;
      for (int j = 'a'; j <= 'z'; ++j) {
        ckStr += j;
        int k;
        for (k = 0; k < names.size(); ++k) {
          if (names[k].find(ckStr) != string::npos) {
            break;
          }
        }
        if (k == names.size()) {
          cout << ckStr << endl;
          return 0;
        }
        ckStr = i;
      }
    }
  }
  return 0;
}
