#include <bits/stdc++.h>
using namespace std;
char a[40][40];
char ans[3];
char ch;
int n;
int find1() {
  for (ch = 'a'; ch <= 'z'; ch++) {
    int judge = 0;
    for (int i = 1; i <= n; i++) {
      if (strchr(a[i], ch) != NULL) judge = 1;
    }
    if (judge == 0) return 1;
  }
  return 0;
}
int find2() {
  ans[0] = 'a';
  while (ans[0] <= 'z') {
    for (ans[1] = 'a'; ans[1] <= 'z'; ans[1]++) {
      int judge = 0;
      for (int i = 1; i <= n; i++)
        if (strstr(a[i], ans) != NULL) judge = 1;
      if (judge == 0) return 1;
    }
    ans[0]++;
  }
  return 0;
}
int main() {
  while (scanf("%d", &n) != EOF) {
    for (int i = 1; i <= n; i++) cin >> a[i];
    if (find1()) {
      printf("%c\n", ch);
      continue;
    }
    if (find2()) {
      printf("%s\n", ans);
      continue;
    }
  }
  return 0;
}
