#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, j, x = 0;
  char c[30][30] = {0};
  scanf("%d\n", &n);
  for (i = 1; i <= n; i++) {
    char temp[605] = "";
    cin.getline(temp, 605);
    int length = strlen(temp);
    for (j = 0; j < length; j++) {
      c[0][temp[j] - 96] = 1;
      if (j != length - 1) c[temp[j] - 96][temp[j + 1] - 96] = 1;
    }
  }
  for (i = 0; i <= 26; i++) {
    for (j = 1; j <= 26; j++) {
      if (c[i][j] == 0 && i == 0) {
        printf("%c\n", j + 96);
        x = 1;
      } else if (c[i][j] == 0) {
        printf("%c%c\n", i + 96, j + 96);
        x = 1;
      }
      if (x == 1) return 0;
    }
  }
  return 0;
}
