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#include <bits/stdc++.h>
using namespace std;
int n;
char a[33][22];
int v1[100], v2[100][100];
int main() {
  while (~scanf("%d", &n)) {
    int f = 0;
    int flag = 0;
    memset(v1, 0, sizeof(v1));
    memset(v2, 0, sizeof(v2));
    for (int i = 0; i < n; i++) scanf("%s", a[i]);
    for (int i = 0; i < n; i++)
      for (int j = 0; j < strlen(a[i]); j++) {
        v1[a[i][j] - 'a'] = 1;
      }
    for (int i = 0; i < n; i++)
      for (int j = 0; j < strlen(a[i]) - 1; j++)
        v2[a[i][j] - 'a'][a[i][j + 1] - 'a'] = 1;
    for (int i = 0; i < 26; i++)
      if (!v1[i]) {
        printf("%c", 'a' + i);
        flag = 1;
        break;
      }
    if (flag == 1) continue;
    for (int i = 0; i < 26; i++) {
      if (f == 1) break;
      for (int j = 0; j < 26; j++)
        if (!v2[i][j]) {
          printf("%c%c\n", 'a' + i, 'a' + j);
          f = 1;
          break;
        }
    }
  }
  return 0;
}
