#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, n, l, j, k, p, a[27] = {0};
  int sum = 26;
  int flag = 0;
  scanf("%d", &n);
  char c[50][30];
  char d[40];
  for (i = 0; i < n; i++) {
    scanf("%s", c[i]);
    l = strlen(c[i]);
    for (j = 0; j < l; j++) {
      if (a[c[i][j] - 96] == 0) {
        a[c[i][j] - 96] = 1;
      }
    }
  }
  for (i = 1; i <= 26; i++)
    if (a[i] == 0) {
      printf("%c\n", i + 96);
      break;
    }
  if (i > 26) {
    for (j = 1; j <= 26; j++) {
      d[0] = j + 96;
      for (l = 1; l <= 26; l++) {
        d[1] = l + 96;
        flag = 0;
        for (k = 0; k < n; k++) {
          for (p = 1; p < strlen(c[k]); p++)
            if (c[k][p] == d[1] && c[k][p - 1] == d[0]) {
              flag = 1;
              break;
            }
          if (flag == 1) break;
        }
        if (k >= n) {
          flag = 2;
          break;
        }
      }
      if (flag == 2) break;
    }
    printf("%c%c\n", d[0], d[1]);
  }
  return 0;
}
