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#include <bits/stdc++.h>
using namespace std;
const int N = 100 + 5;
int n, v[200000], d[200000];
char a[35];
inline void Dfs(int k, int len) {
  if (k == len + 1) {
    int key = d[1];
    for (int i = 2; i <= len; i++) key = key * 27 + d[i];
    if (!v[key]) {
      for (int i = 1; i <= len; i++) printf("%c", d[i] + 'a' - 1);
      printf("\n");
      exit(0);
    }
    return;
  }
  for (int i = 1; i <= 26; i++) {
    d[k] = i;
    Dfs(k + 1, len);
    d[k] = 0;
  }
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%s", a);
    int alen = strlen(a);
    for (int i = 0; i < alen; i++)
      for (int j = 0; j <= 2 && i + j < alen; j++) {
        int key = a[i] - 'a' + 1;
        for (int k = i + 1; k <= i + j; k++) key = key * 27 + a[k] - 'a' + 1;
        v[key] = 1;
      }
  }
  for (int i = 2; i <= 3; i++) Dfs(1, i);
  return 0;
}
