#include <bits/stdc++.h>
using namespace std;
int a[26], b[26][27];
char s[25];
int main() {
  int n, f = 0;
  char c;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%s", s);
    c = s[0];
    if (a[c - 'a' + 1] == 0) {
      a[0]++;
      a[c - 'a' + 1] = 1;
    }
    for (int j = 1; j < strlen(s); j++) {
      if (a[s[j] - 'a' + 1] == 0) {
        a[0]++;
        a[s[j] - 'a' + 1] = 1;
      }
      if (b[c - 'a'][s[j] - 'a' + 1] == 0) {
        b[c - 'a'][0]++;
        b[c - 'a'][s[j] - 'a' + 1] = 1;
      }
      c = s[j];
    }
  }
  if (a[0] != 26) {
    for (int i = 1; i <= 26; i++) {
      if (a[i] == 0) {
        printf("%c", i - 1 + 'a');
        break;
      }
    }
  } else {
    for (int i = 0; i < 26; i++) {
      if (b[i][0] < 26) {
        printf("%c", i + 'a');
        for (int j = 1; j <= 26; j++) {
          if (b[i][j] == 0) {
            printf("%c", j - 1 + 'a');
            f = 1;
            break;
          }
        }
      }
      if (f) break;
    }
  }
  return 0;
}
